I recently had an issue where a user could not login, their session showed disconnected on the RDGateway but I could not log them out vis Server Manager, no session for the user was shown on the Session Host in Task Manager. I used the following commands to find the processes running under that user's account and end them.
Some processes could not be ended but ending those I could allowed RDGateway to unregister the session and by denying logins to the host I was able to steer the user to another host.
To list the users logged in and obtain their Session ID
query session
To log off a session
logoff <Session ID>
To list all processes and find the Session ID related
query process *
To list processes and obtain the PID (process ID)
tasklist
To end a process
taskkill /F /PID <PID>
To log a user off from another server
Logoff <SessionID> /server:<hostname>
