Lock the tax report

Once the tax report has been prepared, ask the user the 'lock' it. After that it can't be modified, no matter what. It will allow me to delete older stocks with 0 value, but not affect the tax report. New tax year, new stocks, new report. Instead of 'locking' it, you could just back up, or ask the user to mark it 'backed up' once their tax return has been lodged.