There is no way I am aware of to block the part number from appearing in a LOV for the Part Number in these records. Blocking is available for elements of Basic Data, but a Part Number is not basic data.

If you are using a Part Status on the Inventory Part that is configured for ‘Supplies Not Allowed’, users can see the part number but cannot save the part to a Purchase Requisition or Purchase Order.

If a Supplier for Purchase Part record is set to Status = 1 (NOT ACTIVE), users can see the part number but cannot save the part to a Purchase Requisition Line or to a Purchase Order Line for that Supplier.

If you are using a custom field on a record such as the Purchase Part or Supplier for PP to denote the part as Blocked, you would need a custom event action to prevent selecting the blocked part number on a PR or PO Line.