CFC has offered you good advice, another way to eliminate them is to place a piece of lettuce leaf in the water. (depends how well planted your aquarium is) this will attract the snails and you can dispose of them that way. However to ensure you don't get them in the first place, and they usually come on plants or planted material. I would suggest that you soak all your plants in strong salt solution for a few moments, then rinse off the solution in fresh water and physically check your plants for eggs. The snail cyst is impervious to any solution and the only way to destroy them is to "squish" them between your finger and thumb. Depending how invasive your tank has become, the only solution to your problem is to maybe strip down your tank-disinfect everything and restart again. Snails are a problem once they become invasive and being hermaphrodite, can reproduce rapidly. A warning-dead snails can decompose unseen altering your water chemistry rapidly.
