On the Ready to Ship page, there is a green bar with a list of all the orders ready for carrier selection. To the far right of every order there is a small icon to remove the order from the shipping queue. Once an order has been removed from the Ready to Ship page it will appear back on the orders page.

From the orders page you can permanently remove the order from shipping easy by selecting its check box, then moving to the More button located between the green and blue bars at the top of the orders page. Once you click on the More drop-down, you can select clear orders. This action is irreversible, use with care.