From experiences written here in the past, writing a letter advising that you intend to take legal action if they continue to pursue the issue of df will sometimes prevent any action being taken.
As to why it works in some situations and not others is not clear. It may have a lot to do with your current status as a JW, particularly, whether you are active or inactive.
If you are active then you could still be considered a JW member and it appears that the Watchtower can decide whether to formally reject you as one. If you are not currently active then it may be that the Watchtower has no authority over you and any so called membership status does not exist. However, your actions could reactivate that relationship. You need to be careful not to acknowledge being a member or even formally disassociating yourself.