Throw it back on his lawn, that's what I used to do. Did it right in front of him once, so fresh it was still steaming, he didn't say a word...
It works very well. Once when a guy walking his dog along our steet allowed him to poop on our lawn and then just strolled on, I went out, scooped it up with a plastic bag, and caught up to him. I tapped him on the shoulder and handed him the bag of warm poop, and said, "Here you go. If you ever need a baggie, just knock on the door."
He never has walked the dog past the house again.
Frank 2135