Net price non editable

Dear Gurus,

My client has a requierment where in the net price should not be editable . i tried to make the settings in screen layout for the PO .With this the net price becomes grade out and not able to edit in item . This net price has picked from the last PO/PIR , but i can still change this price at item level / header level . I want to make this as not editable nor at the header level / Item level .

Shashi.