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.
I'd say they drop to Zero Refresh and become an NPC. They chose to become a monster. This is why having a bigger buffer of Fate Points is important. That said, it would be a short-term problem as a Warden would likely show up and behead the NPC for breaking a law thus making the fact they're an NPC now a moot point.
Rule of Thumb: Don't break the Laws of Magic if it takes your PC in to the Zero Refresh range if you want your Character to live long. Having refresh left to you means you can be redeemed, going to zero refresh means you crossed the line into being a monster.