as mentioned do "ps aux | grep USERNAME" which will out put something like this..
neil 30199 0.0 0.2 5484 1596 ? S 03:11 0:00 /usr/sbin/sshd
then you want to do kill -9 XXXXX where.. XXXXX is the number right after the username in the above example. Obviously the name of the process you're killing is the one at the very end of the above example.