There are two ways through which you can control.
1) The first one and recommended one is, define the Field Level Authorizations (in Tools) and assign the Roles to the Users, so they won't be able to edit the Price Field in PO Line.
2) The second one is through UE DLL. You can define a logic in such a way that, LN will display an error message, if the price origin of the PO Line is "Manual".
Hope the information is helpful. If you still have any queries, get in touch with me on LinkedIn - in/vemavarapusasikanth