You should write the congregation' BOE and tell them you do not wish to continue as a member of that specific congregation.
In the same letter you should threaten to take legal action against the elders if they say anything to mar your reputation with your former congregation associates.
You should make it clear that if you wish to join another congregation you will do so without needing to tell them.
If they say anything or write anything against you, as in a letter to another BOE, you will take legal action against them.
This will prevent their announcing anything about your not being a JW anymore, all you've done is leave that congregation and that's not a DF'ing offense.
For the BOE to take any action against you would be opening themselves to a lawsuit since you've already informed them that you are no longer a member of that congregation.
There are legal precedents that support a person leaving a congregation and suing a former congregation due to retaliation on their part for one's leaving.