Hate to say it, but you might want to remove all your snails, put them in a QT tank, and manually remove them in there. Then, maybe leave all the snails in the QT for a few weeks to see that no more of the bad guys survive. Meanwhile, keep your display tank empty of all food sources these bad guys could eat and hope that they all starve to death with their food source removed from the display tank. That's probably the best way to remove them.