If you start a session and see another user's name and password box, you should contact and ask them to start a session to Sign Out. Only one user may be signed onto a Frame One workstation at a time and disconnecting is not the same as signing off.
If you are unable to reach the other user, contact Frame One Support. We can force a user to sign off, but it carries some risk of data loss if the user hadn't saved their work or minor problems like "stale" bin locks if they have bins open in a shared project. For more information read Share a Workstation.