Hi,
Then, you should keep that option to "no" and when you notify the user by email via the order details page of the backend, before sending the email, you should just set the payment method of the order to PayPal. That way, the user would also see "Pay with PayPal".
Otherwise, you can leave the dropdown and try to remove the line:
$this->values[] = JHTML::_('select.option', $type.'_'.$id, $name );
from administrator/components/com_hikashop/types/plugins.php so that the "none" value is not added to the dropdown, but that might not work.