Hi Jonathan,
That’s an interesting conundrum. I’m surprised TfL can’t/won’t stop the card even if it has a negative balance. Do you know whether the card had a deposit on it? If it did then you probably don’t owe TfL anything. If you haven’t already called the helpdesk then I would do so, and ask them exactly what TfL would like you to do. They’ll probably say that nothing can be done, but ask for a call reference and the person’s name, and that’s as much as you can do.