How to enforce a new buy item on a End-Item Substitution Chain

I'm using an ASCP distribution plan with the following decision rules: use end-item substitution and use alternate sources.

Suppose I have 3 items, all of them are "Buy" items and they are substitutes one to another, it means, they have bi-directional relationship (reciprocal) (A<>B<>C). Each one has its own independent demand. So the planning engine would seek supplies from demanded item or substitute item and use it up as effectively as it can before creating supplies. When necessary, the planning engine may place order to buy  unfulfilled demands. Currently, my DRP has recommended new buy orders for all of the items. Now, suppose the items A and C are no longer sold by suppliers. How could I determine