Truth be told, I have yet to see any biological remedy guaranteed to destroy Vermetid snails; not a fish, not a crab, not anything. If there are only a few vermetids and the tank is small, I have read instances where the hobbyist has gotten rid of them through a combination of manual removal and nutrient control. But once they reach 'plague' proportions, you'll have to let them crash. And they will. Unfortunately, this will take time, hastened only perhaps by the implementation of the same manual removal/nutrient control previously mentioned. Sorry I don't have better news.