[Americanwolf]

Iodine does a good job.....blueberry juice also

[shezza]

Interesting article on the dye pack used by banks to nab bank robbers (1-methylamino-anthraquinone): "How Stuff Works"

[grassboots.org]

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.

[aruanan]

Silver nitrate? But it can be corrosive and toxic. Better to coat it with butyric acid. This isn't dangerous, but it stinks pretty bad.

[The Hollywood Conservative]

What's that stuff John Kerry put on his face last week?

[Old Lady]

The best thing to do is go to Radio Shack and pay $200 for a camera that hooks up to your computer, then position it in a window -- their camera will film endlessly and is not limited to 8 hours or so you would get with a VCR. Film the person who comes in your yard and call the cops.

OR... stay up all night hiding behind a curtain with the lights off, take a picture, and call the cops.

[patriciaruth]

Methylene blue. (Ask a clinical lab to supply you with a Gram stain kit).

Or ask a pharmacist for the purple stuff they used to paint throats with back in the old days.

These are dyes that won't kill your evil twin neighbor, just bust him.

[ArmedNReady]

Gunpowder! It makes a nice black ring around the entry hole.

[UsnDadof8]

I would say whatever it is they use to make those freaky looking blue guys in the Intel commercials.

[elli1]

Hartz Mountain makes a dog repellent called ''Outdoor No''. Supposed to keep dogs from urinating on your landscaping. The stuff stinks to high heaven (or it did when I had some years ago.) You could try giving the area around the sign a liberal dose late in the evening...

[Old Sarge]

Your exotic pets can help.

That neighbor with the pet iguana will be only too happy to provide you with the leavings from the iguana's last meal. Anyone with such an exotic pet will understand...

[DSBull]

Getting the prick on film is the best way. I think that you have them on Criminal Mischief and destruction of private property. Call police and take him to the Pokey..

In Texas here, you can shoot someone who is committing Criminal Mischief at night, but I wouldn't recommend it.
Although hiding in the dark and putting a pellet in their butt would be tempting...

[Tarpon]

skunk shot gel

Looks like this would get the stinker ...

[FourPeas]

How about putting a spotlight on it? Or, put a motion detecting device (see Radio Shack) connected to a spotlight. If you want to get really fancy, connect a camera to the motion detector. You may get some pictures of various urban wildlife along with your liberal thief.

[cwiz24]

Rig your sign with a little alarm that goes off when it is touched. Scare the crap out of anyone...

(Wireless Alarm)

[Doomonyou]

[ShandaLear]

Don't know the name of it, but there this stuff you can get at your local garden store that acts like fly paper. It's a clear liquid you can paint on the sign. Once your neighbor touches the treated sign, it will take him hours to get the stuff off his hands.

[Old Lady]

Whatever you do, don't put a sign in your window -- somebody could break the glass. Also be wary of chaining the sign to valuable trees or shrubbery that could be cut down.

[Tijeras_Slim]

http://www.tanglefoot.com/products/ttcoating.htm

Tanglefoot insect trap coating. Sticky as he!!, odorless, colorless, and non-toxic (liability).

[kajun]

This will stain any skin....