When I first had a look a my girlfriends unsecured wireless router, I found that she had about 6 other people who had jumped onto her bandwidth.
I quickly secured the network and all was good.
I just found this great little link about how to have fun with the intruders instead of cutting them off
Through these router configurations, you can re-direct them to a specific website, or even more fun, you can turn their browsing experience into hell by making their images turn upside down (not the text), or making their images blur.
Check out Upside-down Ternet