I just got an angry call from someone that received my letter. It started "normal" with him saying he didn't want to sell his house and that he didn't like that I had the audacity to ask. No problem. I handled the call with my usual tact, apologizing for upsetting him and asking him for either his name or address so I can go into my database and prevent from bothering him again.
He responded that his name was "John". He refused to give me any further identifying details, saying if I found him in the first place I should have no issue knowing exactly who he is now. I tried to explain that I had a marketing list and that I don't know which "John" he is, so without further information I was likely to mail to him again. He said "If you send me another letter I'm going to charge you with harassment," and hung up on me.
What am I supposed to do at this point? I have at least 50 "Johns" on my list. My list grows daily and I send out letters daily so I have no way of knowing even which "group" he's in. I'm not going to just stop mailing to everyone named John. I'm definitely not going to call him back. Do I just ignore the threat and hope to get more information next time when he inevitably calls again (since clearly if I don't know who he is I can't remove him from my list)?