Given your noted situation, chemicals are out as well as starving them. Your only choices are manually removing them combined with very thorough gravel cleanings. You might consider adding snail eating fish, certain Botias, Loaches, and S.A. cichlids, but that is a big commitment. As for the snail, it's some variety of common pond snail. There are a lot of AA members who are very knowledgeable about snails and will be able to ID them.