There are various methods of removing them, either 'biologically', physically or chemically.
If they are suited to the set-up, the best and most natural way is to add snail-eating fish. The best candidates are usually loaches. Clown loaches are one of the most popular snail eating fish, and usually do a good job. If your tank is not large enough for these (recommend 3ft minimum), the smaller Pakistan or Zebra loach may be more suitable. Certain catfish like 'Dorids' (talking catfish) or banjo catfish will also eat snails.
Even if physical removal daily can never completely wipe them out, this is a good way of keeping the population down. "Baiting" often works - if you place a slice of cucumber or lettuce in the tank at night (weighted down so that it stays on the substrate), the snails will congregate on it and then you can just pull them out of the tank with the cucumber slice. One way to avoid the fish eating the slice is to stick it inside a clean bottle, or beneath an inverted plate.
Hope this helps.
