One thing I've recommended to parents as well is to set up the modem for the internet in your bedroom (if you have cable run it to your room, if it's phone line, same thing, etc.) and add a wireless router. Make all the computers wireless (you can by a dongle/pluginthingy for about $35 here in Canada for computers that don't have Wifi in them) and then when you go to bed you turn off the modem. Won't stop them from using it during the day when you're out but when you're home and especially at night when you're asleep no internet, unless they're really sneaky and hook up a cell phone or steal a neighbours signal or something.