Closing Purchase Orders 
Prerequisites
The purchase order has been partially drawn to a higher level purchasing document.
Procedure
From the SAP Business One Main Menu, choose
Purchasing – A/P 
Purchase Order 
and find the relevant purchase order.In the menu bar, choose
Data Close .
Result
The status of the purchase order changes to Closed. The purchase order itself is not deleted. You can still display and duplicate it, but you cannot change it or draw it to a higher level purchasing document.