SP TN to block/restrict user from edit/change price field in PO

Hi,

How to restrict or block user from edit/change price field in PO.

I found the similar post, but there is no answer on the TN, can anyone please help ?

http://scn.sap.com/thread/3231899

The approval procedure will not meet the requirement as the user is still allow to edit on others field other than POR1.Price

Thanks