Sometimes it is necessary to disable users from logging into a system,particulary when doing certain types of administration work.
To disable user login in AIX, simply create the file /etc/nologin.
When a non-root user attempts to login, the system blocks the login and displays the contents of the /etc/nologin file. So u can also write a message in the nolgin file which will get displayed when non-root user tries to login.
To enable user login, erase the /etc/nologin file.
/etc/nologin file gets removed automatically if server is rebooted.