When we meet vandalism, we block users. Often schools use proxies to accommodate the whole school with access to the Internet. When we block the proxy of a school, the good suffer with the bad.
An idea would be to mark the proxies of schools with details of that school. If we have an e-mail address for a school, we can mail them that at such and such a time abuse was registered from their school. When we have this organised well, there will not be a need to ban indefinitely as we will have a mechanism to deal with it more accurately.
The drawback of such a scheme is that it will cost more time to administer. The benefit is that kids will be given notice that abuse is not accepted by our projects nor by their school.