windows command line logoff
logoff /? logoff [{SessionID|SessionName}] [/server:ServerName] [/v]
query session // command line to see what user is logged on tasklist //shows you who's logged on, what their running, and the Session Name and #
logoff rdp-tcp#1 /server:servername //log off user from the server
of course you can always
shutdown -r /t 0 //reboot the server now!
This is of course useful for Windows Server 2008 core (aka DOS) if you have an RDP session in that has somehow exited the command prompt (and can therefore only disconnect!)