If you would like to review your clients orders before they are to the labs for fulfillment, you may want to consider turning on Order Delay. Order Delay allows you to delay orders for a specified timeframe for your approval and review, before being submitted to the Lab for production.
You can cancel an order before it has gone into production if it is in the "Pending" stage and while within Order Delay. Each option will give you the ability to cancel the order, which will stop it from going to the lab for production, and refund your clients immediately. Once an order has changed to "Processing" status it can usually no longer be cancelled or changed.