I think the best solution is to simply take the computer away. Like you said, that's group punishment, but there's a reason that group punishment is so effective. When I was a teenager, I didn't care what my parents thought about me, and so if all they did was ground me, I would just focus on getting through it and making sure I didn't get caught again.
But when you use the punishment to indirectly alienate the child from his/her peers, that is much more effective. Then they have to face not only your anger (which they don't really care about), but also their peers' anger (which they do care about).
