Is there a way to prevent automatic creation of PO from requisition for some items only?

We use autosourcing so our POs are created automatically after requisition import.  The create document workflow is kicked off and automatically releases POs based on the supplier, site, item set up in the blanket agreement in our ASL.  Now, we are faced with a requirement to prevent PO creation for top 10 requisitions with highest cost.  Our supply chain management team would like to manually release POs for these requisitions.  We already tried to set SUGGESTED_VENDOR_ID, SUGGESTED_VENDOR_SITE, and SUGGESTED_VENDOR_SITE_ID to NULL in the requisitions interface but POs were still created.  Is there a way we can stop automatic PO creation