Then mulitple condoms are not for you in this case. Far too risky.

There are many techniques for delaying orgasm in men, some of them discussed above, but I am not sure any of them actually stop orgasm once it passes the point of no return. Once you reach that point there is little that can help you and even if you withdraw so that you do not cum inside her you have still cum and that defeats the purpose. Squeezing the head of the penis may help and has very BDSM connotations if she does it to you. Usually this used to be used as a trick to prolong the sensation of orgasm in men (and mean that she is more likely to orgasm if the man has problems with premature ejaculation) rather than stop it completely but if you withdraw and do this then stop all sensation on the penis for a while (maybe give her some attention with a dildo, vibe or your fingers/tongue for a bit) then your penis will subside. You may even be able to get it back up to erection and have another go for a few minutes untii you approach orgasm again and you can then repeat. It takes a lot of practise to get this sort of thing right - both the timing of the withdrawl (you both need to know your responses accurately) and the squeezing of the penis head. Ice cubes may help to speed up the process of losing the erection.

I am not sure you can effectively prevent orgasm without losing the erection - the very fact that the penis is engorged is a stimulating thing so that needs to be lost, if only a little, before you can stop. So any process will need to involve a cycle of erection, penetration, approach point of no return, withdraw, go slightly flaccid, achieve erection, penetrate and so on. Also be aware that each cycle will take less time to reach point of no return. Plus, if you are allowed an orgasm in such a state, after many cycles that orgasm will be intense...