Take an empty water bottle, cut the top off. Piece of lettuce or other vegetable matter in the microwave for 30 seconds, put that in the bottom of the bottle, put the top in backwards. Snails will be drawn to the lettuce, and can't get back out. You may have to do this several times, replacing the lettuce, to get them all out.
Put it in at night, remove in the morning.
You can also try OLD pennies, they have a high copper content. New pennies won't work.