Use the Cancel Style function to completely remove the SKU from all customer orders. The system will report which order contains the SKU. Inventory > Cancel Style
Use the Substitute SKU function to replace the cancelled SKU with another. Advanced > Utilities > Substitute SKUs
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If you wish to keep the SKU in the system catalog, and only remove it from the orders, Advanced > Utilities >Remove a SKU from all Unreleased Orders > Enter the SKU > GO. In this case, you may want to mark the SKU as "Stop Selling".
Please note that none of the listed functions affect purchase orders.