Disablin login for system accounts

Hy everyone!
I'm trying to lock out system accounts on my Tru64 UNIX machines.
Wherever I look, it says only to put Nologin or *Nologin in password field of desired account in /etc/passwd.
But it doesn't work. I still can su to that accounts. There are some accounts that already have Nologin in their password field and to that account I cannot su.
What am I doing wrong?
Do I have to do something else besides adding Nologin in /etc/paswd?
I used vipw for editing passwd file, in case someone asks;)

Another approach is to change their shell to "/usr/bin/false".

Also, the shell is supposed to exist in "/etc/shells" for it to be considered valid.