jdrakeh
Front Range Warlock
As I hinted at above, if you're really concerned about that kind of issue, you could introduce something like magebane- an achemical potion that dulls a spellcaster's ability to cast- or spellchords- enchanted ropes or other bindings that prevent hostile casting.
They seemed to work pretty well in the fiction I'm lifting them from...
Threat of death seems to work just fine for me. Only because the players know I'll follow through, though.
