Thanks, also what I was looking for. To adjust it the 'clean' way you can override the Mail class:
class Mail extends MailCore
{
public static function Send($id_lang, $template, $subject, $template_vars, $to,
$to_name = null, $from = null, $from_name = null, $file_attachment = null, $mode_smtp = null,
$template_path = _PS_MAIL_DIR_, $die = false, $id_shop = null, $bcc = null, $reply_to = null)
{
// Don't send an email a credit slip is created (not configurable in backoffice)
if ($template == 'credit_slip') {
return true;
}
return parent::Send($id_lang, $template, $subject, $template_vars, $to, $to_name, $from, $from_name, $file_attachment, $mode_smtp, $template_path, $die, $id_shop, $bcc, $reply_to);
}
}