How to kick someone out of Linux box?

Kick out a user:

pkill -9 -u username

Kick out a user from one session:

skill -KILL -v /dev/pts/*

http://www.cyberciti.biz/faq/howto-kill-unix-linux-user-session/

http://www.cyberciti.biz/tips/howto-linux-kill-and-logout-users.html