Are you looking to log them off of a Terminal Services session, off the networked shares, or off their workstation that is a member of the domain? Basically which one?
1) Terminal services will let you disconnect a session after a certain period of time has elapsed. Set this up in the users account.
2) If you want to disconnect them from network shares (to keep files from hanging open) you need to use group policy and go to the Computer Configuration, Windows Settings, Security Settings, Local Policies, Security Options. In there (depending on the client that you set the group policy up on, XP pro will show different options than 2000 for example) you will find: Microsoft Network Server: Amount of idle time required before suspending session
3) Not sure on this one, and I would like to know also.