I used to have a real problem with snails and, more specifically, slugs, too. While researching soil components, I discovered that Diatomaceous Earth is a natural insecticide, used in its powdered form to kill bedbugs, mites, pillbugs, snails and slugs, among other things. Diatomite, commercially available as a spill absorbent, acts very similar to products like Turface, and it's significantly cheaper. Long story short, I no longer have any problems with slugs or snails. I will often find them on the bottom of pots, but never any evidence they've been on my plants. The only caveat, you have to make sure the diatomite is from a freshwater source, usually by looking up the product's Material Safety Data Sheet.