Vendor Hierarchy and purchase orders

Hi,

We have activated the vendor hierarchy and it's working in purchase order, but we want to control the same. for Example, we have assigned X and Y as goods suppliers in Vendor Master of A in partner function.

and system shows the same when we create purchase order but we can skip it or change the name of goods supplier in purchase order which we don't want to do,

So pls help us how can we control the same, we want in purchase order, we can assign only a goods supplier which will be either X or Y not beyond that.

Pls advice.

Thanks

Sandeep