If you don't believe in using chemicals which are usually metaldehyde based products, another option is to use beer in shallow saucers and let them drink themselves to death. Alternatively go out with a torch after dark when the snails are starting out on their slimy travels. Collect them in a bucket and dispose of humanely or inhumanely. Eventually the population will be decimated. You can also search for them in their hiding places during the day, typically under or behind pots, containers, rocks, bricks, window sills etc. If you've added any stones or similar at the bottom of flower pots to prevent the drainage holes from being blocked and a cavity is formed, they can hide in there also.