I had a client that sold novelty signs. He had one that said "Here lies the last dog that pooped in my yard." Kind of cute but makes the point.
My preference would be to find out who he is, catch him doing it, then walk out and hand him a bag to pick it up with. Or put something up that has baggies attached saying "please pick up after yourself..."
What I would probably do if he doesn't get the message is make sure you know who it is, then put a bunch of it on his doorstep.
