I had a lot of problems with hermits eating my snails as well. I've seen the blue legs and scarlet hermits do this to astrea snails, and then use the shell. The way I solved this problem was by buying large mexican grazers. They are bad about knocking things over that are not well placed though, so if you buy them beware. When these guys die, the smell is horrendous too if you don't get them out immediately. Its quite a large snail, and they all seem to have voracious appetites.