Windows Server 2012 - Force Terminaluser to logoff

I'm not sure what you mean by "force the logoff of that user via GUI" so you might be referring to this, but I'll throw it out there anyway:

My GUI method is to open Task Manager on the server, go to the Users tab, then right click the user's session and choose Log Off.

If that fails to actually log off the session or their session isn't listed there, then you can try opening Computer Management on the server, go to Shared Folders, then Sessions. Sort by username, and then right click on each record for this user and clickClose Session`.

That should handle it. If it doesn't, then something is hung up/buggy that would require a reboot of the RDS server itself.