Disabling shell access doesn't fully lock a user out of a server. They can still login via ftp/pop/imap/smtp servers if they are running on the local server and are checking local user authentication. Below you will find instructions on how to fully lock a system users account.

In this example we will lock the system user johnsmith

/bin/passwd -l johnsmith

In this example we will unlock the system user johnsmith

/bin/passwd -u johnsmith