As I see it, they have the right to discuss the religion as much as they please. However, if they are trying to coerce you into joining or into going back, they have gone beyond that right
Certainley! As long as it's not discussed with YOU. Now witnesses do not get it very well. It is your responsibility to have none of it - to leave the talk, the hall, the memorial, all of that shit. Never discuss it again with them - you can get your information here or anywhere on the net if you want. i.e. "Oh, dad's car broke down when we were going to the hall". Answer: "You've got to get rid of that old clunker. Oh, did I mention that I'm starting Yoga? I'm so excited" not "oh, did you get a ride to the hall with so and so..." Keep it about YOU, not them. They WILL bait you into a discussion with their pathetic cult lives. Have none of it.
Writing the letter, letting them know that IF they want a relationship with you, then it is up to THEM to leave all that negative rottenness to their "friends" to discuss. "I want to live my life happily, without negativity, and I will have none of that nonsense in my life. If I choose to get strong enough to attend meetings again, I will. But until then, you must respect my boundaries, or I will initiate boundaries that will result in us not having a relationship with you anymore. " i.e. fader comment.
It's no different than having an intervention with a person addicted to drugs. These are my boundaries, respect them or you are out of my life like cancer, until you change your life or, in this case, respect my boundaries.
Boundaries boundaries boundaries. Get them, demand them, and don't talk about their pathetic cult lives, ever. It's abnormal, ridiculous, and socially unacceptable.