Smear poison ivy or caustic chemicals on the edges of the signs (while wearing gloves of course).
Hidden cameras are a good idea, too.
In southern NH where poison ivy is the state plant we just put our signs in a good patch of it. Or nail them to a tree hosting the vine.
Sometimes though quite a bit of old axle grease from the barn smeared on the stake holding the sign is a good idea. It makes it easier to hammer the stake in to the ground they say...