I had a neighbor who would let his dog out while he stood on his deck and watched. The dog would come over in MY yard and poop. He'd simply let the dog back inside without picking up it's poo.

One night I took his dog poo (after witnessing for the umpteenth time), put it in a CVS bag, tied it up and placed a note on it that said, "I believe this belongs to you. Please PICK UP AFTER YOUR DOG!!!" and left the bag on his deck.

He got the hint.