General question about dealing with illegal PDL's

I closed my checking account and sent C and D letters to all of them. Some of them have left me alone, but PDL Yes keeps harrasing me. I have told them several times what I will pay and only by MO but they will not listen. They just keep calling me and threatening legal action. "First National Security Legal Department" or something like that is what they call themselves. Of course, the number you call back is PDL yes. So what od I do if they never provide an address? I'm not paying any other way, I don't trust them