How do you write a derogatory credit letter?

You would only be able to write a derogatory letter if you a creditor who reports to the credit bureaus. If someone owes you money, you can go to court and file a judgment. This would show up on someones credit report, showing 'you' as the plantiff and the debtor as filed against. The judgment would remain on a credit report until the judgment is paid or falls off the credit report in seven years.