One of those GD camera phones.
I imagine, I am just guessing, most people have one? Anyways if YOU do,
Show them a picture of the back of a truck with a trailer hitch and say "Hey look at that hitch...."
Even if they ARE gone it would still be funny. Maybe show them a photo of a door with an "Exit" sign above it...
And why would they do this to you if you are letting them use your property? Have they not heard of "Don't poop where you eat"?
Anyways even if people ARE asses about this, look at it this way - they will get over it pretty quick. They may or may not like you after but it will quickly become old news and they will want something fresh.