Lightly boil a slice of vegetable (squash/zucchini) or raw cucumber and put it in the middle of your tank. Wait 20-30 minutes. Take it out with the mass of snails feeding on it, and there you go. Snail population, gone!
If they're getting out of control and reproducing too fast, you're probably putting too much food into the tank to begin with. Eliminate their food source, and they'll stop breeding.
Assassin snails will eat other snails, but you'll still need to remove the old shells. You may end up needing to feed your assassin snails if they clear out the whole population, though.
Just a few tips.