Disconnecting RAS Sessions

Hi I want to automatically disconnect user after they have been on for 30 minutes. Our server is used just to sync email and some users abuse it. I am aware of the registry key that boots after X inactive minutes but I want to boot them even if they are active after 30 minutes. Anyone know a way to do this without using 3rd party software. This is an NT 4.0 SP6 server using digiport and Standard NT RAS Service