I rub my signs with poison ivy. It’s free from my back yard. Then I look for neighbors covered in Calimine Lotion.
Just drape some poison ivy all over the edges of the sign.
It will dry out, of course, but will still pack a punch
for a good lesson without being fatal. Rubbing the sign
with the ivy might also work; but the dried leaves won’t
be all that obvious - until later.
Put some fresh dog doo all around it. Coat it with stink juice. Rig it with a hotel door alarm. Or go full out - alarm it with hi power floodlights and very loud alarm siren and have camo cam.