Hot fence charger.
Should be easy enough to place steel mesh screens on front and back, with wires going to a hidden car battery nearby in bushes (or inside). Positive on one mesh side, negative on the flip side. When the sign is grabbed, the two screens make contact and burn the hands. Not enough of a charge to kill, just enough to scare off the intruder. Double down by coating with poison oak. Triple down by placing dog crap around it that can't be seen at night.