The best idea is to rub the signs with poison ivy... That’ll fix em.
I like the way you think! Presumably the perps arrive after dark. Spread feces (crap) on the grass in front of the signs. Embed a set of trip wires some four feet back from the feces. When the perps fall, they'll be nose deep in it. Hear the screams and call the cops.
OMG - that’s such a good idea !