This doesn't seem to be working quite right.

Helpfile suggests that you can't take people's own spells/supps, but I regularly take supps from the enemy that used them (e.g. protection from paladins). Protection, for some reason, is the easiest to take. Often it then causes me problems because I have protection from evil still in effect when my protection from good expires, and can't renew protection from good.

Other than that, it very rarely does anything. Maybe 1 in 100 dispelled effects, but I guess that's by design.

Maybe it could work a bit better on the supps/spells it is meant to, and not at all on the ones that it isn't meant to?

There is some special logic regarding protection that was breaking this. I fixed it so you shouldn't be able to steal it any longer.