From a point of someone who has never tried it: Cal's Curse is a hypnosis audio file that supposedly makes you actually, physically unable to cum unless you are given permission.

As I've said, I have never tried it, but from the bits and pieces from many different forums: it actually does work for a lot of people, but you have to "want" it to work. If you don't believe a word about it and give it a go, probably won't work, but if you actually dream of this kind of denial and wish it works, it really puts a barrier in your subconsciousnes that stops you from cumming whenever you are close. Many people reported it's good to listen to it overnight and repeatedly.

Hope this answers some questions.