Selectively implementing a "No PO, No Pay" policy

Summary:

My client is new to using purchase orders and would like to ease into it by only using them on a small scale before considering a 'No PO, No Pay' policy.

Content (required):

I have seen the option to 'Hold Unmatched Invoices' on the supplier site, so I can suggest that they enforce the use of PO's only for those suppliers.

Setting it up based on specific categories was also mentioned, but I am not sure how to implement this as there are no categories on AP invoices. Any ideas on how this might be done, or if there are other ways to restrict the requirement of PO's?