Great Idea, until the neighbor kids walks up to your sign just to take a peek. This looks like a lawsuit waiting to happen. I don't know if booby-trap laws would apply or not, but I'd be careful:
It would be better to set camera surveillance and then report them for theft.
You make a great point...
Any "booby trap" device that harms ANYBODY, whether they are good guys, innocent passerbys, or even the sign destroyer, WILL GET YOU ARRESTED, AND WILL GET YOU SUED.
It's just a sign folks... How much does it cost??? A couple of bucks? Lets not lose perspective... They're tearing them down BECAUSE THEY'RE LOSING AND THEY KNOW IT...
Dont sink to their level, and don't give them a chance to ruin your life, which they'd LOVE to do...