Paint the borders of the sign black, then after the sign is set in place, brush on black axle grease mixed with graphite on those border areas. That shit is a bear to get off and everything he touches will be smudged. No one gets hurt but it sure screws things up.
Apply Tanglefoot to back of the sign...
Tanglefoot 15 oz. Insect Barrier-300000625 at The Home Depot
It's a non-drying sticky adhesive that makes rubber contact adhesive seem like Elmers paste glue.
Alternative to the axle grease and graphite suggestion... Anti Seize
Permatex 80078 Anti-Seize Lubricant with Brush Top Bottle, 8 oz. : Amazon.com : Automotive
Anyone who has ever gotten even a miniscule amount of it on their hands knows what a mess it can make.