There was some conjecture that caffeine was toxic to them, and that's what gave me the idea to try the coffee. Studies with lab-produced caffeine solutions - much stronger concentrations than in coffee - had mixed and questionable results, which make me thing that either 1) it's something else in coffee that is effective, or its combination with caffeine, or 2) my experience and observation was a huge coincidence, and the snails died/left for another reason.