How to register a supplier so they can create POs but not negotiate with the supplier

Summary

How to register a supplier so they can create POs but not negotiate with the supplier

Content

My ct don't want to allow to create a negotiation with the supplier, but they want to allow to create POs for the supplier.

How can we enable this requirement?

I have unchecked address purpose "RFQ or Bidding", but it did not work. I still can select the supplier site at RFQ supplier tab after unchecked it.

Is there a way to enable this requirement?

== Business Background ==

It is little bit complicated to explain. Simply put, it is like this.

1. Want to create Direct Material POs for the supplier via interface from the external system.