Ruined orgasms are another one of those ironic and self contradicting kinks. People with this fetish enjoy not-enjoying their orgasm. Usually what it is they are enjoying is your control and dominance over their "ultimate" form of pleasure. This can also be one of those fantasies that people only want to experience once, and then have the threat of future ruined orgasms give you leverage during play or punishment. The fact that you can and are willing to do something so cruel is often a powerful turn on and motivator for your sub. There are however, some who like the idea so much that they actually have stronger orgasms when they think you're "ruining" their orgasm. I'll go over both; how to more completely ruin an orgasm, and how to make ruined orgasms more enjoyable.
The most completely ruined orgasm is one where, as soon the sub reaches the point of no return, he receives no further stimulation. You completely withdraw your hands from him, and leave him with nothing but air. But this can be trickier than it sounds. Having your sub tell you when he's just hit the point of no return not only feels like cheating, but it gives your sub the power to control how ruined his orgasm is by letting you go a little longer before he tells you he's there. The best approach is just to have your sub tell you when he's getting close to going over the edge. Test him by stopping right when he says he's close. If comes at this point then it's possible he was trying to get a less ruined orgasm from you. Punish as you see fit. Then try going just a few more strokes after he announces he's getting close. It might take a couple tries to figure out roughly how much more stimulation he can take when he's close. I suggest going only a stroke or two longer each time as to get the most fully ruined orgasm. I also suggest randomly stopping immediately after he says he's getting close just to keep him honest. This is a variance on the orgasm control game I wrote about last month. To judge how ruined an orgasm was, the best approach is to compare how much the sub came compared to normal. If there's a lot of cum, then he's either secretly enjoying it, or you've gone on too long after he's reached the point of no return. Subs with a strong fetish for ruined orgasms often enjoy them more than they would a simple vanilla orgasm.
If you look around for methods of ruining orgasm you'll find a lot of techniques other than the simple hands off approach I described above. When first learning about ruined orgasms I was put off by the multitude of options. In the name of science I set out to try as many different approaches as I could find, but almost immediately I realized that these variants don't ruin an orgasm more, they just make it more "fun" for you or your sub. In the same way some people like being "punished" with things they enjoy, some people like having their orgasms "ruined" in a way they enjoy. These people want to think they're giving up great pleasure to please you, but they secretly want enjoyment from it as well. Realizing this makes ruining orgasms much less complicated, if you're out to be cruel or completely dominating, this is the ideal approach. If you're out for fun for you or your sub, try out some alternative approaches. While I still suggest you start by getting your sub to that point of no return, if you're just doing it for fun you can always just wait till you see him start to come before you do anything. There's also an off chance one of these variants will ruin an orgasm more than the hands off approach, but in general they make the experience more enjoyable. The best way to find out is to experiment.
The most common variant to a ruined orgasm is to restrict the flow of cum. The most common way to do this is when your toy hits that point of no return, you squeeze his cock either at the very base, or just under the head. While not necessary, if done tightly enough, little to no cum will come out. Another approach, sometimes called thumbing the orgasm, is to put your thumb over the top of his cock head as you would to control the flow of water from a garden hose. This can also be done with a finger or even tape. Less cum is often associated with less enjoyable orgasms, so this gives the appearance of a completely ruined orgasm. The discomfort from the semen backing up has the potential to make the orgasm less fun for your sub. If you are worried about this you can let more of the cum escape. But usually the fact you are touching him more than offsets any discomfort.
Physical pain can be added to a ruined orgasm. If you are rough enough it could take some of the pleasure out of the orgasm, but once a sub has hit that point of no return, his pain threshold goes way up. For most subs this will cause a much better orgasm than the hands off approach, and for those who like pain or CBT, they may even have amazing orgasms. The most common method for mixing in pain is flicking the head of the cock or slapping the cock itself. Balls are an option too; slap them hard and/or squeeze them tightly as he comes. Another "painful" method is palming. This is often done with post orgasm torture, but with subs who are sensitive enough, this can be painful before orgasm as well. Palming is just using your palm on the head of a well lubricated cock. For most, this will simply make it more pleasurable. I suggest you try palming your sub before it gets to this point to see how he reacts in a normal situation. While it varies from person to person, pain will often improve the orgasm for the sub.
There are a couple off the wall approaches that don't fit into the above categories but are based on distraction. One is to have ice ready and ice his cock and balls once he hits that point of no return. I've heard it will numb away some of the pleasure, but there doesn't seem to be enough time to numb anything. In the end it's just a distraction through temperature shock. There's seems to be a bunch of approaches which are designed to surprise the sub like this. These include, at the point of orgasm, playing a loud sudden noise, shining a bright light in their eyes, and pouring lemon juice into their mouth. I've heard suggested choking or covering the sub's mouth and nose. The idea is to scare them into a more fully ruined orgasm, but depending on how scared the sub actually is by this, it might not even be an ethical approach. I've actually seen tops trying to one up each other with the surprise they've induced in their subs. Another form of distraction is repulsion. This includes things such as putting stinky socks on their face (assuming they don't like that), playing gay porn (assuming they aren't gay), or showing them pictures of their family members. In the end, distractions tend to loose their effectiveness quickly, and you have to keep coming up with new distractions. Still, unlike the previous mentioned methods, distractions stand a good chance of actually ruining an orgasm more completely, if that's your goal. If not, you can always choose "distractions" your sub will like.
Which brings me to the mental aspects of a ruined orgasm. If a sub has a ruined orgasm fetish, then the ruined orgasm will be better if the sub is told about it ahead of time and visa versa. The anticipation will make play beforehand more or less enjoyable depending on the sub's interests. Tying up a sub so he can't "finish" a ruined orgasm also adds to the orgasm, especially if you make it clear it's so he can't do anything about his upcoming ruined orgasm. This works even if the sub is well behaved and wouldn't do such a thing anyway because it makes the already dominant act feel out of their control and more absolute. Another trick is to get the sub to try and hold back the orgasm himself. This gives a more completely ruined orgasm and can take the fun out of ruined orgasms even for those who have a strong fetish for it. If your goal is a more completely ruined orgasm, you can tell your sub not to come or otherwise try and motivate him to try and hold back. This just feels mean to me, and since only well behaved subs will actually try to hold back when instructed, it rewards misbehavior. And that's something you should try to avoid. But if you're using it as a punishment or just really want to take any possible fun out of it, this will sometimes do that for you. Finally, ruined orgasms can potentially leave your pet feeling unsatisfied or horny. Sometimes the sub will be unable to do anything about it, while other times the sub will be able to have another orgasm right away. Several ruined orgasms in a row can be quite fun and frustrating at the same time. It depends on the sub and the orgasm, but is something to keep in mind and watch for.
Any top who has "lost" or otherwise had a ruined orgasm is probably confused by this fetish. To better understand a sub who is interested in ruined orgasms, see if it's the controlling aspect of a ruined orgasm your sub likes, or if he likes the idea of being disappointed and wants you to use the threat as leverage or punishment. And it can be both. If your sub isn't interested in ruined orgasms but you are, then using it as a threat or punishment is probably the best option. While different approaches fit different subs better, it's important to pick something that sounds fun or interesting to you. There's no need to go through extra work you don't enjoy or pass up something you might. There's also no reason to limit yourself to one particular technique, feel free to mix and match. Don't let it be a chore for you.