How to deny or block user login in linux

Deny user login by locking out account

Pass -l option to passwd command. It is used to lock the specified account and it is available to root only. The locking is performed by rendering the encrypted password into an invalid string and by prefixing the encrypted string with an !.

Syntax

passwd -l {username}

passwd  -l rajesh

Unlock account or allow login

To allow login use passwd command as follows:

passwd -u {username}

Passwd  -u rajesh

This is the reverse of the -l option - it will unlock the account password by removing the ! prefix.

/sbin/nologin shell

/sbin/nologin displays a message that an account is not available and exits non-zero. It is intended as a replacement shell field for accounts that have been disabled or login is blocked.

Example: Deny login for rajesh user

Type the command as follows (login as root user):
# passwd -l rajesh

             or
You can also change shell to /sbin/nologin:
# usermod -s /sbin/nologin rajesh

Example: Allog login for rajesh user

Type the command as follows (login as root user):
# passwd -u rajesh

              or
You can also need change back shell from /sbin/nologin  to /bin/bash:
# usermod -s /bin/bash rajesh