I have a couple of suggestions.
After rain and at night pick them off by hand. Then feed them carrot or corn meal for 2 days until their poo looks like what you feed them. Starve them for another 2 days and make some tasty peppered snail out of them. By picking them off by hand you can get ahead of them and take the reproductive adults out of the system.
Other one is to cut the bottom off some some plastic bottles and put a little beer or coke in the bottom, they will be attracted to it and take it and die in the container.