Timed logoff from user workstation

On an NT network with users having Win98, Win 2000 Pro, and winNT workstations.
Is there a way to automatically force a logoff of a user if his station is idle for 20 to 30 minutes?
The reason is, we have a group set up for Internet acccess. Thereare some very remote workstations that are used by shop supervisors. If they get up and leave there station that leaves there PC open to anyone that wants to use it. I know that it can be password protected, but some of these people have a hard timeremembering there name. I would like it to be auto. forced.