The Users section of the Retrieva preferences window is split into three vertical sections. The right most section is concerned with connected users.

If you are logged in as the Retrieva administrator, you can disconnect users that are currently connected to the Retrieva system.

To do this, select the user you wish to disconnect in the Connected Users List and click the Disconnect User button. You will be asked to confirm the disconnect and then the user will be disconnected from the system.