Dang, I'm fresh out. Oleo capsicum powder will have to do.
Use poison ivy or some other caustic plant oil on the sign. Another is a semi-buried board of nails around the sign.
There is an equine product called Rap Last. It’s made to keep horses from messing with their blankets or leg wraps. This stuff is unbelievable. Edge your sign with it and they won’t be back. If they get it on their hands and then touch their eyes, nose, mouth or other sensitive areas they are going to burn for awhile.