How to Restrict to only Pick the Approved Requisition Line into the Standard PO ??

Hi ALL,
I know how to created the Standard PO from Approved Requisition. Here what i am looking forward to know is to restrict the Buyer so that he is unable to have another line into the PO except the Requisition Line.

For Example : Requester raise the Requisition containing 2 Lines and Requisition then Approved from the Approval Authorities too. Now buyer using Autocreae Functionality creating the PO from the Requisition, so as we know that once the PO is generated we can further more line on the same PO. But here based on the client we want the Buyer is unable to add another Line except the Requisitions one ??