Originally Posted by
Saint-Just
By the rules it isn't. First Fatigue from labor recovers at point/10 minutes or faster, so keeping the caster at exactly 0 FP is impossibly hard, and with 3 points you can try to do something in some situations (Create Door, 10-yard Teleport, Lockmaster on one lock, Levitation for 1 minute, and many more options become available if you manage to recover say 6 points). Then usually there is an option to get some spells at 0 cost (organizing an escape with only those may be too hard, but unlimited Message usually also more than jailers are willing to allow their prisoners), and finally there is an option to get into negative FP or burn HP to power the spell (but jailers cannot normally keep people at negative FP, it will kill them in days).
For mages all you need is Drain Mana or Suspend Magery, but other magic-users are harder. Divine-powered guys are especially hard to block within the "generic fantasy" options.