They are pretty much immune from the salt trick, but they are not immune from Lye, Find their trails, and put around 2oz Lye per Gallon of luke warm water in a pail (the luke warm water will disperse the Lye better), then with gloves on, soak a rag and spread the Lye around the plants that you want to save. The African Snail's reaction to Lye will rid them of your garden area almost immediately. It inhibits their ability to feed, and lowers their movement rate to almost 1/3. And once the Lye enters the GI Tract, the snail is dead.