If you don't secure your wireless router, it an almost certainty than at some point or another, someone will use your network.

If you don't really mind, or have a spare wireless router and a linux box (doesnt hurt to have some *nix skill either), you can turn your cyber-thiefs upside down! This page describes some very easy steps to serving your thieves upside down content, or even blur just enough to be annoying. This page has my vote for the day, I'm loading up a Linux distro as I type, gonna have some fun!

Upside-Down-Ternet