I don't drink coffee either so I can't give you any first hand experience, but I've read that caffeine is also toxic to plants. You may want to research that before pouring unused coffee grounds on your cherished plants. Used grounds have much of the caffeine leached out in brewing so it's not really a concern with those. I'm sorry that I don't have the answers for you regarding caffeine's effect on slug or plant health, but hopefully someone will.
