Do
ps aux|grep username
This will list all the PID (Process IDs) created by the user
Look for "-bash" (or whatever your shell is) in the last column, should be two open since you are logged in twice.
Note the PID (second column from the left) of the shell then do
kill -9 PID, where PID is the PID number you found.
That'll kill the users shell. Only way I know how