Nothing you can do about it. Hermit crabs will attack and kill snails to steal their shells, so they're not doing it because they're hungry. You can help a bit by buying a bunch of different sized shells and tossing those in the tank. Some hermits will use the empty shells instead of killing snails, but lots of hermits will continue to kill snails. They seem to enjoy it, almost.
Lots of people don't keep hermits for this reason. For people that do keep hermits, it's just a fact of life that you have to periodically replace your snails.