This is my signal, and nobody else's!

Don't you just hate those complete and utter bastards walking around, pirating WiFi signals? Don't you wish you had a way to do something about it?

Well, now you do! Ani-Wi-Fi paint, it's called. From Christopher Null:

The idea is simple: Use a special paint on walls where you don't want wireless to pass through (say the exterior of your house). The secret is mixing aluminum-iron oxide particles in with the paint. The metal particles resonate at the same frequency as Wi-Fi and other radio waves, so signals can't pass through the thin layer of pigment. Outsiders would simply be unable to access your wireless network, just as you, inside the house, won't be able to interlope on anything beamed on the outside.
Developed by the University of Tokyo, the paint is said to be the first that can block radio frequency in higher spectra where Wi-Fi and other higher-bandwidth communications occur rather than just low-frequency wireless like FM radio. Most Wi-Fi technologies operate at 2.4GHz; the Tokyo paint can reportedly block frequencies all the way up to 100GHz, with a 200GHz-blocking paint now in the works

The paint isn't just of interest to those concerned about wireless leaking out of the building. Movie theaters have long been interested in finding a legal way to keep cell phones silent during screenings. Electronic jammers that actively block wireless signals are illegal, but passive materials that prevent wireless signals from getting through are not. Since the wireless-blocking paint can also block the lower-frequency signals that cell phones use, addled mobile junkies would have no outlet for reaching the outside world.

Now, speaking as one of those pirating bastards ( hey, a guy's got to check his email sometimes ), I have to say that this idea is still pretty cool. Particularly as I don't really like to give what I get: if I had a wireless network, I don't know that I'd want to share it with the world to download the type of pornography ( or whatnot ) that gets traced back to people by the authorities.

That being said, one has to wonder if the possibility that your neighbor might be some hacking, pirating porn-freak is high enough to warrant such measures. I guess it depends on the situation, but unless your neighbor looks suspiciously pale and has an early-in-life case of carpel-tunnel syndrome, I wouldn't worry about it.