SAP DBM - Job order fields Non editable after Billing.

Dear Experts, I would like to understand how to make the fields read only(Non editable) in DBM Job order once the billing is completed. The action - ORD_CLOSE & ACTIVITY_CLOSE is maintained in the action control for relevant access sequence ( for Event BILLING_CREATE). Also subsequent event ACTIVITY_CLOSE is maintained for Event - BILLING_CREATE. Request you to help me with suggestions. Regards, T.R.S.Praveen