To get rid of them on clams I just placed the clams on eggcrate or a PVC coupling. Every day, I would remove each clam and scarpe down the entire shell with a toothbrush and run it under a stream of tap water. Obviously you don't want to expose the clam's mantle to tap water, so be careful. It's important you do it every day to get eggs, the tiny ones you don't see, etc., and to do it to every clam. After about 2 weeks I stopped, and they never came back. I never had an issue with them going after snails.