This worked for me:
controllers/admin/AdminOrdersController.php and comment out the following code as shown:
/* @Mail::Send(
(int)$order->id_lang,
'credit_slip',
Mail::l('New credit slip regarding your order', $order->id_lang),
$params,
$customer->email,
$customer->firstname.' '.$customer->lastname,
null,
null,
null,
null,
_PS_MAIL_DIR_,
true,
(int)$order->id_shop
); */
I am using Prestashop 1.5.4.0