If you have any available, maybe cheaper to rub the sign down with poison ivy. They likely won’t make the connection but it will sure ruin their day. 100% natural.
I considered your approach but found it too much. What if sign stealing moron goes into anaphalactic shock? I came up with a solution. Sticky fingers for the people with sticky fingers.
Most campaign signs are stolen at night, so smearing the back side of the sign with roofing tar works like a charm.
The first thing they will do is wipe it on their pants, and if they have to get into a car it will be all over that.
Best part it is that it wont hurt them in any way, so no lawsuits.
Works well. I bought a Mr Juicer last year and actually ran quite a bit through the processor. Got a few drops and mixed with vaseline...