Physical access was the only means we found for controlling it. For the first kid this worked reasonably well. He's 19 now and a comupter genius through and through, so his computer was like his arm. He couldn't deal with not having access.
For the second(15 yo), not so well. Most of his time online was social and since we took it away he started sneaking out at night to hang w his friends. Eventually brought home by the cops. Total backfire.
Be careful which direction you go.