Drop Ship Orders - Prevent Cancelling the order line

We have a drop ship supplier that fulfills orders on our company's behalf.  We are using the eBS Drop Ship functionality (Order Management/Purchasing) to execute the fulfillment of our orders.  Our current setups allow for us to cancel an order line after it has already been sent to the Purchasing module for communication to the Supplier for fulfillment.  This obviously causes problems if we have already sent the fulfillment request to the Supplier.

I know we can implement Processing Constraints in Order Management to prevent cancelling order lines under specific scenarios.  I see to Validation Templates that show promise of us leveraging to prevent the cancelling of an order line if we already run the Purchase Release program.