Thanks for the help. I'm in the process of setting up a server computer, mainly to allow printer sharing and file sharing, but I'm going to do some of this stuff once that's done.
The problem with explaining this sort of thing is that unless it's actually going to harm them, they don't care.
We're changing ISP, though, to one that won't charge more if we cap.
Personally, I think the data cap for each computer would work best. Set it in such a way that certain computers get so much, and any other computers (i.e unwanted connections) get none. That way if she screws up once and goes over, she'll know not to in future.
He alone, who owns the youth, gains the future.