http://www.linux-noob.com/forums/index.php?showtopic=1166
Had to look this up again after two users in our dev server got hacked. I originally thought `skill -u username` would work, but it didn't.I had to manually kill each sshd connection of the hackers after changing the passwords of each user. I then proceeded to make sure they didn't leave any rogue programs behind and topped off with putting in a few rules for iptables to keep logins at least 30 seconds apart. This wouldn't prevent a determined script kiddie, but it should slow him down a bit.