order quantity reduced after delivery already created

A sales order with a material w/ item category TAP is exploded for 10 boxes

The Subitem, w/ item category Ztan is created as a subitem for 10 boxes

A delivery is created for the Ztan item for 10 boxes.

A CSR can enter the sales order, and change the quantity for the TAP item, and reduce it to 5 boxes

This change will affect the ZTan item to 5 boxes

This happens even though the delivery is already created for 10 boxes. Thus the goods issue and invoice occurs for 10 boxes

How Can I prevent this from occuring? I have tried completion rules for the Ztan (B & C) but that hasnt helped.