Removing Anonymous FTP access is as simple as enabling the access. Removing access does not delete your public_ftp folder or its contents (you should not delete this folder anyway), but it does prevent anyone from accessing that folder through FTP.
To remove Anonymous FTP access:
Click on the Ftp Manager button on the home page.
Click on the Anonymous FTP Controls link.
Remove the tick from the Allow anonymous access to ftp://ftp.yourdomain.com tick box or the Allow anonymous upload to ftp://ftp.yourdomain.com/incoming tick boxes.
Click on the Save Settings button.