How to enable a prohibited group file in vsftpd?

Issue

In vsftpd, there is a file called ftpusers and it is located in /etc/vsftpd/ directory. The text file ftpusers contains a list of users that may not log in using the File Transfer Protocol (FTP) server daemon. It will typically contain a list of the users that either have no business using ftp or have too many privileges to be allowed to log in through the FTP server daemon. Such users usually include root, daemon, bin, uucp, and news. The format of ftpusers is very simple. There is one account name (or username) per line. Lines starting with a "#" are ignored.

By default vsftpd only creates and uses ftpusers file for blocking users.