Snails are hermaphrodites, meaning they can reproduce by themselves without a partner. If you want to keep some of the snails, your best bet will be manual removal of the smaller snails. You can create a trap by tying zucchini to a string and a weight and put it in the tank overnight. In the morning you'll have a bunch of snails feasting on it, which you can take out and dump.

But if I were you, I would get rid of all the snails and keep the tank clean another way. You can easily do this by buying a clown loach or yoyo loach. They will devour most/all of the snails pretty quickly.