This is an interesting concept, in that their powers are as much about mental discipline as anything else. Do you have an antagonist in mind? There are many different opportunities available. If your stoicists gather their power by their ability to ignore physical pain and maintain emotional control, what would motivate them to come into conflict with one another if that is your intention? If the goal is to be calm and absorb an attack, you might run into trouble if both sides are defensive. If they are both passivists, who would throw the first punch? What about introducing another class that derives their power by causing or feeling pain and emotion? Sort of an anti-stoicist. This could lead to some psychological disorders and sadomasochistic tendencies. But maybe I'm getting away from what your intentions are.

What would be really interesting is the psychological impact this sort of power will have on your characters. The biggest threat to their powers seems to be their own level of discipline and ability to remain dispassionate. That may make them think about what happens if they start to enjoy the pain. As you said they will lose their abilities, which might weigh heavily on their minds. You may want to introduce characters that have experienced this loss. Perhaps they are simply powerless, or maybe they would exhibit symptoms of withdrawal.

Another thought. What about using chemical pain suppressants like morphine? That could open a whole new bag such as addition, dulled senses, and death.

This is a neat idea. Best of luck to you!

a whole list of 'em here:

dreaded tvtropes link

Awesome find. Exactly what I was looking for! Thanks and thanks to everyone else who have offered suggestions!

Small correction: Copywrite is what a copwriter does. It is the profession of writing scripts for advertisements.
Copyright is the legal ownership of a work, which is what I think you meant.