We've added a feature specifically for canceling items in a customer sale. You can create a "cancellation" in much the same way as you create a credit note, or shipment for a sale. Just navigate to the "Cancellations" area at the right side of the sale details page and click the "+" button to bring up the Create Cancellation dialog.
Canceling items will stop WOAH from suggesting that you ship or invoice for those items. It will also decrease the calculation for required production and, if resulting in surplus inventory, increase the quantities in the "available to sell" report.
Changes to credit note behaviour
Please note that in the past, creating a credit note had the effect of reducing the outstanding quantity to ship. This is no longer the case and credit notes have no effect on shipment/production calculations. You now need to create a cancellation to stop WOAH from suggesting you ship the items. If you have already invoiced for the sale, you'll probably want to create a corresponding credit note after cancellation, and apply credit note to the open invoice.