1) Edit the pam file for the service you want to control, in this example i modify ssh pam control file, located in /etc/pam.d/sshd
Add this line
account required pam_nologin.so
2) Create the /etc/nologin file, just do "touch /etc/nologin"
This should disable the login from ssh. If you want to disable the login from terminal, modify the /etc/pam.d/login file.
3) To re-enable the login just remove /etc/nologin
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source:http://linuxpoison.blogspot.com/2008/06/135781758012382.html