how to force-close unattended linux systems

[Paul Johnson]

In our computer lab, there are some users who walk away from their
computers for hours.  I'd like to automatically close down those
sessions so that other users do not have to wait or hit
ALT-CTRL-Backspace.  I don't find anything while googling because I
don't know the magic words, I'm afraid. gdm is the login manager and
the gnome screensaver installed.  I just need some little addon to the
screensaver that logs out systems if they aren't used for a while.
Similar to the way the laptop power manager closes down idle systems.

Got any advice?

-- 
Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas