Do you not like the snails? are they disturbing the levels in your tank? If you dont really mind them (meaning, if they arent casuing TONS of ammonia, although i know they cause tons of poop and cleaning) just let them be. I know it seems you have millions but there is only so much food for them to eat in there. their population is self-limiting. you will still have the occasional explosion in population but many will not survive- there is only so much food in there for them to eat.