Someone suggested putting the yard sign in a fireant hill, and put "Tanglefoot" on the sign and post. It is messy, sticky, and smelly. Someone else suggested fish hooks on it. They come after dark, they won't see the sharp hooks. Also staple the sign to the post and leave the staple sharp on the back side, don't fold them in smooth. Another way to nick their fingers. I plan to rub poison ivy on my sign.
What a practical humane idea...love it!