Best way to kill snails/slugs: get a very low height plate, and fill it with beer to about an inch height. Snails/slugs are attracted to beer and will come out and drown into the liquid.
Sounds weird, but you want to attract them out of their hiding places, and get rid of them. And this simple effort actually works.
I think you would have to use so much that it wouldn't be worth it. Try planting plastic tumblers, filling them with beer, the snails, slugs are attracted to the beer, crawl inside and drown.
The other option is to put gravel around the plants, the slugs/snails hate it.