Fast and fun:
#su - user
$kill -2 -1
That kills everything run as that user; if not, try -9 instead of -2. You can also pick through their history before you do the kill. You should disable logins for the account before you do this, in case they are fast with the login. As usual, I am merely suggesting things so please use your own discretion. I know far more about how to break these things than I should. IBM Certified -- AIX 4.3 Obfuscation