Spontaneousspontaneous generation
Ifif this is a medieval world, that operatescan operate on a medieval understanding of how the world works, than rather than approaching this problem withhaving to conform to a modern view, then the snails can arise spontaneously from the earth, as at least some people duringin some parts of the middle ages believed many small animals did. They literallythey can't be hunted to extinction because even if none are left, more will still appear.