UM/Hotspot how to disconnect on limit reached

I think the subject explains it, but I ‘sold’ a school a RB with UM/Hotspot to replce a linux hotspot solution that they had on a PC that was starting to give problems. They allocate MB instead of uptime for each student.
The problem is now that sometimes a student can get 3 or 4 times his/her allocated amount before they manually disconnect.

Has this ‘problem’ been adresses yet, i.e.:

How can I force a session/user to immediately disconnect once their transfer limit has been reached?

Thanks in advance
Ekkas

You may want to keep tabs on this thread: http://forum.mikrotik.com/t/how-can-i-impose-a-limit-on-the-download-in-megabytes/31534/1