How do I deny access to user account? Do I need to use /bin/false or /sbin/nologin to refuse a login?

The /sbin/nologin command politely refuse a login. It displays a message that an account is not available and exits non-zero. This is prefreed method these days to deny login access to account. You can use it as follows:
# usermod -s /sbin/nologin userName

The /bin/false is old method which does nothing and always return unsuccessful code. You can use it as follows to deny login access to existing user:
# usermod -s /bin/false userName

More About /etc/nologin File

If the file /etc/nologin exists, login will allow access only to root user. ther users will be shown the contents of this file and their logins will be refused. This is used when you need to deny login access to all users except root account. Just create /etc/nologin file and you are done:
cat > /etc/nologin
Sample ouputs:

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A Better Solution

Lock and unlock user accounts using the following commands:
# passwd -l userName
To unlock it again:
# passwd -u userName