Snails are a parasite? Because they eat things that we try to grow in our gardens? Humans aren't exactly saints in this regard.... but I have an old Victorian walled garden - the walls are like blocks of flats to all the snail families in the area!
The best ways I've found to deter them are copper strips (which they won't cross), or vaseline which they don't like to cross either, barriers of eggshells, holly leaves, ashes or coffee grounds, or buy cheap lettuces or melons and leave them out as decoys and move them in the morning well away from all your precious seedlings!
Elaine - I agree, my heart sinks when you hear that crunch, all life is sacred and everything on the planet has a right to live as best it can. I try not to go into the garden after twilight, or only with a torch if I do to weave my way around them!