On the BT 2wire router each machine can be given time slots when they can/can't access the net or specific sites blocked. You would have to log into the routers admin. screen and then look for access or security settings. If the teenager has wireless and there is an unsecured network near by they could be on the net without using your router. I believe many use the pay as you go dongles to get round adult interference in their online social life's.