Put her pc on its own switch with a digital timer on it?
You seem very negative about all the suggestions made she isn't going to be god even if she can play guitar hero.
If she connects using cat 5 get rid of that, if she connects it back up just cut the wire, may sound a bit harsh/drastic but there is quite a severe problem. Same with wireless block her mac address or switch it off. Only way to get around that if she doesn't know the router admin password is to get devious and reset it etc, if that happens its time for pc access to be taken away all together.
Port blocking 37** whatever it is will also work, try forwarding it to a non existant ip address on your network.
Don't know about you're router but i can limit access to any pc, wired or wireless, for every day of the week. If yours doesn't it may be worth searching for something that does