In DFRPG, you have the 7 laws of magic, which if you break you automatically have to add it as a power. Lets say we have a focused sourcerer who's used 6 of their 7 refresh, and they break one of the laws. Do they add the power and drop to 0 refresh, or are they forced to give up something to keep them above 0?
From Your Story, page 232:
So strictly by the rules, they add the power, drop to 0 refresh, and become an NPC. But that's no fun, IMO. The PCs know this possibility, so if you warn them, and they do it, then it's a big deal. So play it as such. There are a few options that I can think of readily.
And those are only off the top of my head. In the end, I think that anything that causes a PC to leave the game should be played up for drama, and not used lightly, no matter what the rules say.