If you want to thin snail populations can try leaving a lettuce leaf in tank at night close to where you see them. Take leaf out in the morning and it should have them still feeding on it.

To wipe them out your best chance is to get a couple of assassins in there. Not Ninjas ; but these guys: Anentome helena. They are ruthless snail exterminators and when snails are gone will consume food leftovers in tank bottom. Good luck !