For PS 1.7.7.8 the new file is located in
/src/Adapter/Order/Refund/OrderSlipCreator.php
Comment out this piece:
@Mail::Send(
(int) $orderLanguage->getId(),
'credit_slip',
$this->translator->trans(
'New credit slip regarding your order',
[],
'Emails.Subject',
$orderLanguage->locale
),
$params,
$customer->email,
$customer->firstname . ' ' . $customer->lastname,
null,
null,
null,
null,
_PS_MAIL_DIR_,
true,
(int) $order->id_shop
);