Disable ftp account of a specific user

If you want to disable ftp account of any user, you should follow these steps.

The process does not prevent user to log into shell prompt. User can log into shell prompt as well as execute the commands but he can't use ftp services.

If you are using debian or ubuntu. Run following command.

$ echo username >> /etc/ftpusers

If you are using centos or fedora. Run following command.

$ echo username >> /etc/vsftpd/ftpusers

The user will not be able to access ftp account.

If you do not want to use command line. Open file /etc/ftpusers or /etc/vsftpd/ftpusers (according to your distribution) and add username in the list.
To Allow access :
Just remove the username from the file ftpusers.
Restart vsftpd.

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Enable ftp account of root

By Default, root is not able to access ftp because it's name in the list of /etc/ftpusers or /etc/vsftpd/ftpusers
Remove root from the list.
Restart vsftpd.
root will be able to log-in using ftp.

If you are using CentOS, fedora or RHEL and still you are not able to log in as root in your ftp server.
Add following line in the file vsftpd.conf
userlist_deny=YES
Remove user root fin the file /etc/vsftpd/user_list and restart vsftpd.
root should be able to log-in using ftp.