I'm so sorry that she can't just leave you alone.
At this point, I would advise NC completely. All your letter is going to to is let her know that she's getting under your skin, which is what she wants.
Unless you're interested in filing a restraining order, which might not be a bad idea, treating her as if she doesn't even exist is a better signal than acknowledging her in public or sending her a letter. She's not going to hear you, and it will only fuel her to nose into your life more often.
The next thing she gets from you is a letter from your lawyer. Crickets, otherwise. Believe me when I tell you this is how to save your sanity.