[struggle]

Just rub poison ivy all over the sign.

[mumblypeg]

Paintball mines are good, too—they explode and splatter paint on the perp.
You can buy them or DIY.

http://www.hustlepaintball.com/mines

http://www.instructables.com/id/Build-a-Paintball-Mine/

[Hambone 1934]

LOL....LOL...LOL.....

[RightField]

I put pine tar on the edges of my Bush signs in 2000 after my first two sets of signs were stolen from my yard. I found the pine tarred signs laying in the yard one day. Don’t know who took them but I had the satisfaction of knowing they had some nasty cleanup to do.

[rlmorel]

Spray it with pepper spray each day.