From time to time, we will need to disable users.  This may be prior to the permanent deletion of the user account or just as a precautionary step.

As root, you can easily disable a user account by

# usermod –L <username>

Or you can just add an ! in front of the 2nd field (the encrypted password) in the /etc/shadow entry of the user.

To re-enable the user, you can

# usermod –U <username>

This will unlock the user, or simply put, remove the ! in front the password field of /etc/shadow.

If you would like to permanently delete a user, you should use

userdel –r <username>

to also remove their home directories.  Note that in a typical administration, you would usually keep the home directory in case of the need for their data.