Prevent changes to ship-to and vendor address in Process Purchase Orders

Hi SRM experts,

I have recently used BBP_UI_CONTROL_BADI to affect the displays of fields and buttons in the shopping cart. I now need to change the displayed fields in the Process Purchase Order screens (Vendor Address and Ship-To address) to prevent edit. I've debugged in method BBP_PO_UI_CTRL and see screens 1000, 2200... 3000 etc. but can't see the screens and fields I need to affect. I suspect that I might be at the limitation of using the BADI or am I looking in the wrong place?

Can anyone help?