[Chaos]

Right now, the main reason why Snatch Spell looks unstoppable is because it's a 3M move with no real strings attached. I think it should be treated the same way as Create Shade: It requires 3M minimum, but consumes all momentum on use. That, or like Cara suggested, give it a 6M cost. With a Snatch Spell that will eat up a turn, opponents are capable of counteracting its use. If the FP cost seems a tad cheap, then bump that up as well (but not to some 50+ FP).

On the side, Spellthief is a hard counter/nightmare to anything that's overly reliant on Spells. It's the same way with guns; if you focus solely on a certain thing and your enemy/enemies are built against it, your loss is close to guaranteed. This is the major flaw found in anything that goes 'screw variety, I will be very good in this one thing'. The prime method to stop that issue is, you guessed it, variety. In fact, if you mixed up your Spells with other Skills, I'm pretty sure that few would be so keen on keeping their Snatch Spell up.