Project T&M Billing without Proforma - How to writeoff rows?

Hi All !

For some project we don’t need to have a proforma invoice for our T&M billing. In this case the PM create directly a AR invoice.

But we face a problem, in case we have one or more row to writeoff, how to writeoff them easilly(writeoff and hide it for the customer)? 

If we delete the row, it become unbilled and will be recalled at the next invoice.When we want to have a reminder it’s fine , but if we want to completly remove the row it doesn't do the job.

Thanks