Not too long ago I was at my Petsmart talking to one of the workers who has helped me quite a bit and he was saying that pond snails can also come in on the water from the fish bags. They had a couple of their tanks that had no live plants and they were just crawling with pond snails. Not sure how true that is, but that could explain how you got them.
There are a couple options for getting rid of them. You can get some fish that eat snails like loaches, you can stick a piece of lettuce or cucumber over night then in the morning remove it (the snails should be all over it eating it), or my favorite solution get some assassin snails and let them take care of the problem. =) hope that helps!