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 Setup anonymous FTP access link in the FTP Menu area.
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 Submit button. Click on the Setup anonymous FTP access link to return to the Anonymous FTP window to confirm that Anonymous FTP has been removed correctly.