you could go the starve them out route , get someone with a qt or yourself if ones running to babysit your clam/s and without a food source like anything will die out in time.they dont need sand , and on top of that they are very good hunters , in order to starve the food source has to be gone.
http://www.reefpestsolutions.com/#!pyramid-snails/c1ds1
http://archive.reefcentral.com/forum.../t-639037.html