It's not possible to reject individual Ads within an order. If an order has been submitted for approval, you won't be able to partially approve the order. You can Request Review and provide details on what requires amendment in the Ad so that the Supplier can make the necessary changes and re-submit or you can reject the entire order and the Supplier can submit a new order without the individual Ad.
When rejecting the order, you'll be able to provide feedback on the reasons for rejection.