Logout other users while logged in to Administrator
Open Activity Monitor, and find the loginwindow process running for the user. Force quit it. You will then have to authenticate as an admin user. This will log out the user and terminate all of his running processes.
WARNING: This will cause the other user to lose any unsaved work, and could also cause corruption of any files the user has open. Proceed with caution.
via Apple – Support – Discussions – Logout other users ….
