If you wish to remove FTP User accounts, then you can accomplish this by following this tutorial. You cannot remove the primary FTP account as that account the same as the cPanel user account.
1. Log into your cPanel account.
2. In the "Files" section, click on "FTP Accounts" Icon.
3. Under the "FTP Accounts" list, Click on "Delete" button.
4. Click on the "Delete" button and your FTP Account will be removed.
Warning: If you select the "Delete the Users Home Directory" option, then data inside your FTP User account will also be removed and will be unrecoverable unless you restore it with a backup. You may also have prime42 restore a backup for you. If you elect to use this option, ensure you understand which folder(s) will be deleted in the process.