Use the quser command to find the session ID, then logoff to log the user off as follows:
C:\Users\swilton>quser /server:_SERVERNAME_ USERNAME SESSIONNAME ID STATE IDLE TIME LOGON TIME administrator rdp-tcp#19 2 Active 1+00:31 22/10/2015 11:11 AM swilton rdp-tcp#43 3 Active 1 6/01/2016 12:19 PM C:\Users\swilton>logoff /server:_SERVERNAME_ 3 C:\Users\swilton>quser /server:_SERVERNAME_ USERNAME SESSIONNAME ID STATE IDLE TIME LOGON TIME administrator rdp-tcp#19 2 Active 1+00:32 22/10/2015 11:11 AM