Try using some anti-theft fasteners. You might have to get your local hardware store to special order them. If you screwed them into wood you could screw them up and take a file and file the heads downso no screwdriver would work. Or if you used nuts and bolts take a punch and hammer and beat the threads up on the back side of the bolts. A thief gennerallywon't take too much time to steal things, at least I'm sure they wouldn't on your signs. Andwhen your ready to replace them it won't really matter how bad you tear them up, their no good anymore.