How to prevent users from creating a manual PO for an drop ship Org?

How to restrict users from creating a manual PO for an drop ship org? If form personalization is the only option, do we have an example of where this was achieved? In our business scenario, even though users are instructed to not create manual PO's to an drop ship org, due to attrition we end up having users creating manual PO's for drop ship sales order. These manually created PO's are also received manually immediately. Finance team also made a payment to supplier for those manually created PO's.  Hence we want to prevent users from creating the manual PO to