I had the same problem when I helped administer a LAN connected to a WAN, when I was in the Navy. As far as we could tell, there is/was no way to force them to log off. However, what we did is we went around and put a screen saver on every computer, clicked for it to be password protected, and set it up to come on after 5 minutes of inactivity. After the screen saver activates, it requires them to unlock the workstation. I realize that this is not logging them out, but it does help with security, and it is annoying, which may help for them to remember.
Woody