Changing Header/Item condition after billing has been done

Hi,

I have an issue in which the sales order is incomplete but invoicing has been done. On seeing the change log I found that new condition type has been added after generation of invoice.

Now the user wants that entire header/item condition screen should be greyed out after creation of billing document. can it be done through config or user exit is the way,

My apologies if I posted at a wrong thread.

Thanks

Subhadeep