The text file ftpusers contains a list of users that may not log in using the File Transfer Protocol (FTP) server daemon. This file is used not merely for system administration purposes but also for improving security within a TCP/IP networked environment.
The ftpusers file 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.
If your FTP server daemon doesn't use ftpusers, then it is suggested that you read its documentation to find out how to block access for certain users. Washington University FTP server Daemon (wuftpd) and Professional FTP Daemon (proftpd) are known to make use of ftpusers.
The format of ftpusers is very simple. There is one account name (or username) per line. Lines starting with a # are ignored.
passwd(5), proftpd(8), wuftpd(8)