To solve this issue, just logon to the terminal server with special session called the console session and log off the other unused users.
To do this, simply type the following in a Start -> Run or Command Prompt.
mstsc /v:00.00.00.00 /admin
or alternatively you could to the following
mstsc /v:00.00.00.00 /console
Note: 00.00.00.00 is the IP address of the terminal server.
Once logged into the server, right click on your task bar and select ‘Task Manager’ from the list. When the ‘Task Manager’ comes up, select the ‘Users’ tab. In this window, you will see all of the users currently connected to the server. Select a disconnected user, and then click the ‘Log off‘ button at the bottom to clear it.