I was interested in this also, but there is no simple clean way to do this that I have found. I ended up just looking out the window and finding some guy parked outside with a laptop. Knocked on the window, short exchange, pulled the plug on the modem for 5min. and he took off. Or you could just endcrypt your network, but I haven't been able to get WPA to work on a 1Ghz TiBook, so I have to shut it off.
BTW, FIRST POST ON MY NEW POWERBOOK! ITS THE 15" AND I'M GLAD I DIDN'T WAIT.