How to set up the system to not allow to create a PO without a Preq????

Hi Everyone,

Can anyone have an idea on how to set up SAP to NOT allow Purchase Order creation without referencing it to a Purchase Requisition?

My requirement is obviously to always create PO's with reference to a Preq but also when the PO is created, no changes should be allowed to that PO.

Thanks in advance,

Kind Regards.

Julio