Here's how to easily fix an issue where either a remote RDC session is stuck, preventing you from logging onto a server, or you want to log a remote user off of a remote server session for any reason:
First, check the session number with qwinsta:
QWINSTA /server:YOURREMOTECOMPUTERNAMEHERE
Write down the session ID.
Then use the logoff command:
LOGOFF YOURSESSIONIDHERE /server:YOURREMOTECOMPUTERNAMEHERE