You would need to customize the Discount Coupon Order Total module ot_coupon.php
I believe the section of code you want to change would be:
Code:
$this->output[] = array('title' => $this->title . ': ' . $this->coupon_code . ' :',
'text' => '-' . $currencies->format($od_amount['total']),
'value' => $od_amount['total']);
NOTE: removing that from the code will remove it from display on the order in the Customers ... Orders ... and it will remove it from the Customer's Account when review the order ...