Prevent Buyer from receiving for Expense and Service PO's

Summary

How can we prevent buyer from creating receipt for PO with expense, service items?

Content

We have customized buyer role by removing the receiving function and created a role for receiving function to segregate the duties. However, Requisition preparer can enter suggested buyer same as the requester while creating the requisition. PO gets created with the suggested buyer entered in requisition as buyer (given the user was setup as Procurement Agent). As requester receives Confirm Receipt notifications for expense items, buyer (same as Requester, Preparer in this scenario) becomes the receiver. We are thinking of putting a field level validation in place for suggested buyer to not accept the requester (if the field level validation is