Please anyone can tell me what is the procedure to perform the required modifications in configuration files to disable a particular user???
Depending on the OS - the usermod command can lock accounts. You can also change the login shell to /bin/false or something.
Most Unix/Linux OS
Lock account:
passwd <username> -l
unlock account:
passwd <username> -u
vipw
and edit the password entry belonging to that user. Either (1) remove the line, (2) change the shell to /bin/false, or (3) change the password field to "x".
There are other possibilities that are OS-specific, as Frank said. In Linux, you can look at the files in /etc/security.