[Kawakaze]

Good idea about the camera. Why dont you make a sign your self. Make the background black with anti vandal paint (also sold as anti-climb paint), use white vinyl letters. You will know who rips it down because their hands will be stained black for weeks. If it persists I would consider getting metal signs and an electric fence battery! Just kidding, but if you spray paint the sign to your fence it will just get painted over.

Maybe someone is letting you know they dont approve of your dogs, or even the breed itself. Good idea calling the police, if someone gets injured by your dogs and no signs are up, you are liable. Also keep the reciepts for the signs they arent cheap, and make the fool pay you back.