How to stop Billing docs to get cancelled once Debit/Credit memos are created

Dear All,

Please guide me how to stop Billing documents to get cancelled thru T-code: VF11 once subsequent Debit/Credit memos are created in the system.

At present, Original Invoice (billing type F2) created Debit memo (billing typr ZBL3) thru copu control but still User can cancel original Invoive F2 thru VF11.

As per user requirement, once subsequent DMR/CMRs are raised, system should not allow to cancel previous billed F2 invoices.Please suggest how to do it.

Thanks & Regards,

Saurabh