I think I can actually create a contract, prospectively, by sending a certified letter to them (I know who they are and where they live), offering the use of my yard for dog feces deposits, at a cost of $100 per visit. Stating that they can accept my offer by using the yard.
Then take pictures. Because NOW I've got offer & acceptance, and that's enforceable in court.
The alternative is to post a sign on my yard, "Dog Poop, You Pay $100 Per Visit, Place Cash In Slot." (You'd need to bolt something* with a slot in it nearby, and I'd make the sign bilingual.) Run a surveillance camera and a hard drive, so you'll have pics for the small claims action.
Then they can pay for the fence in my yard...
I have enough sense not to put myself in jeopardy in any manner, and seriously, I would never harm anyones pet. I have a dog of my own, and the more I get to know people it seems in this day and age, the more I love my dog.
Besides, I have always felt, "you mess with my dog, you mess with me", and pretty sure others feel the same way..
But I understand the old way of thinking in the country, I'm just not that guy.. I see more of an opportunity.. whether for money, or for funny... LOL