Just had an idea. A friend of mine had a tens machine, which is a device that causes electrical interference to the spine to reduce and confuse the nerve signals of pain during labour. Apparently it's not the most comfortable feeling in the world, but it's better than labour pains.
It just occurred to me that one could be used to interfere with signals in the nervous signals during orgasm. If it successfully inhibited climax, a sub could be instructed to switch it on just as she was about to climax and hence inhibit her own orgasm, either during sex, or while masturbating.