Some snails are really resistant to things that should kill them. My apple snail's been through copper treatments a few times and he's none the worse for the wear. The snails need something to eat, so if they're multiplying too much, you're feeding too much. Some snails is good, in that they'll eat excess food, but too much food means too many snails, so reducing feeding is the first best step.
Loaches were already mentioned - unless you have a really big tank, I wouldn't get clown loaches. A suitable loach will keep the snail numbers under control, but might not wipe them out entirely - there'll probably still be some snails eating excess food and feeding the loaches.
If there's a whole lot of them, put some lettuce or cucumber in the tank overnight. Check it early in the morning, and it should be covered in snails. Take the lettuce and throw the snails out with it.
If you do want to entirely eradicate them (which, IMO, is only necessary unless they're destroying plants), I'd use a combination of the above methods get their numbers as low as possible, and a snail killer only to finish off the last few persistent stragglers. If you poison the whole lot at once, you risk an ammonia spike as they all start to decompose.
