You can specify the date written off on the invoice record. If 'claim tax back' is set to 'no', it has no effect on any reports (it just no longer shows up as unpaid and is moved to the bottom of the list and crossed out). If 'claim tax back' is set to 'yes' AND your tax summary report has 'Include Unpaid Invoices' set to 'yes', it will appear on the tax summary report if the date written off is in the selected date range (there is a whole separate section of the report showing written off invoices). If 'Include Unpaid Invoices' is set to 'no', it does not appear on the tax summary report at all, as it never would have been included in the first place.