I guess it depends on how you intend on using the moss. If you're going to set it up terrestrially the snails should just die off. If its going in a "pond" of some sort you can just pick up one of many copper based solutions aquariums sell to soak in. But actually the snails never really pose a problem in aquariums unless there is too much detritus, which would cause them to over-populate. They rarely feed on live plants.
Dustin
Dustin