The snail crawls in, but can't make it back up the slippery sides. Collect up any snails, put them in a bag and stick them in the freezer for a few hours. If you do put snails in the freezer though, make sure the bag you use is securely tied up first. If so, collect in a clean container with a firm lid and deliver while still alive and fresh. The smell repels them, and it doesn't hurt the soil.
Iron sulphate is cheap, non-toxic, and easy to handle.
It is a widely prescribed iron supplement for people suffering from anaemia. There's no doubt that either is effective, especially if you can directly spray the molluscs themselves. The spray seems to act as a contact poison, so if the animals are wetted or have to cross a sprayed area like a leaf, they die. Copyright 1992 by Greg and Pat Williams. Dispose of contents.