The best way I've found to deface the signs of stubborn spammers is to have the sign fold over on itself so that it is completely obscured. Normally the signs are much too rigid to fold over, but with a few specific cuts you can make it happen.
To do this, first make a vertical cut through the sign on both sides of the stake. Only cut from the top to halfway down the sign. Then on the backside of the sign, from where the vertical cut ends, make a horizontal cut through the backside layer. Only cut through the backside layer of the sign. This leaves the frontside layer intact and the sign will fold over.
I prefer this method since I think it has a much cleaner appearance than cutting part of the sign out. What you end up seeing is the back of the sign which is usually a solid color.
#3. "RE: Clean way to deface sign" In response to Reply # 2
It probably takes less than 10 seconds to make the cuts. What I do now is make the two vertical cuts, then pull each flap forward to make the horizontal cut so that I don't have to walk around to the backside of the sign. It's pretty quick. If you happen to have any signs lying around, practice on that and you'll see what I mean.
#4. "RE: Clean way to deface sign" In response to Reply # 3
I tried your method on a couple of signs one day, and the next day the spammers had "fixed" it by folding the flaps back up. Be sure and remove part of the phone number so that the sign is truly un-usable.