Don't let them do it. There is actually a disease associated with that practice. I am a pet sitter in the south Denver area, there is an explosion in the rabbit population, so "bunny beans" are everywhere. Some client's dogs came down with giardia like symptoms (diaharria, etc.) and had to be medicated. I tell them "no" firmly, if there is poop in the yard, it is harder to keep them away. If you find a poop before they get rained on or sprinklered, you can brush them in a dustpan with a small broom.