I like using coffee grounds for slugs, their distant cousins. It is very effective when placed around the base of the stem. They cannot pass the barrier. Many people will tell you that adding this will lower the ph of your soil. I think this a little dramatic. The grounds are relatively weak. Unless you're actually pouring your mug of coffee into the soil you should be fine. The plants really only need a teaspoon or two each of used grounds to deter your mollusks.