A constant problem for us as every neighbor within shouting distance has cats that freely roam the neighborhood. Subtle hints don't work as cat owners, like most people who bring havoc in to the lives of others, are completely oblivious to the fact. Obviously one doesn't want to shoot a cat, cocoa shells (in the garden, not in a shotgun) work if you don't mind your garden smelling like chocolate. Scooping the mess with a shovel and tossing in to the offender's yard is very satisfying as well. Good luck!

David E.