Dye Pack explosives? Such as what’s done when a bank robber steals money, then he gets coated with bright blue and pink paint.
Get a very large box that is open on one end, a long stick and a rope. Tie the rope to the stick, then use it to prop one end of your box up and over your sign. Take the other end of the rope and drag it to the bushes were you will be hiding. When thy come to steal your sign, pull the rope.
These people are stupid, I’m sure this will work...