Making Billing document conditions page non editable ( grey mode)

Hi Friends

Pls suggest me the procedure where I want to restrict billing document (VF01) by making all the fields in grey mode in conditions tab of the billing document, so that no changes are allowed at in the billing document ( like prices & discounts remain the same as in order ) creation and all data and will be the same data as in the sales order.

Regds

rajiv