They wrote to me twice, unsolicitedly. After the second time, I sent them a "nice" reply. They didn't contact me anymore.
Here's the letter.
Dear Sir or Madam,
I do not normally respond to unsolicited promotional correspondence, but as this is the second time you have attempted to write to me in an unsolicited manner, I think it would be useful to give you my views on your letters.
I am personally aware of a case in which a follower of your religion, who was born into a Jehovah's Witness family, left your community, whereupon the family broke off contact. To what extent it is emotional blackmail for an adult person in such a situation to have to practice the religion of the parents or else suffer exclusion/break off contact is a dilemma that active members of Jehovah's Witnesses can probably judge much better than informed outsiders. In any case, I deeply disapprove of the undermining of family relationships caused by religiously motivated break-off of contact, and I do not wish to belong to any religion that advocates such precepts.
Accordingly, I respectfully request that you refrain from sending religiously motivated letters to my person in the future and remain,
Yours sincerely,