Thanks for your response. As I see things, you can definitely do this. I propose, though, that a way to do that is simply say to them, "From now on, time allows me to call you once every two weeks rather than once a week." Or whatever the exact parameters have been and will be. On this approach, you are neither apologizing nor explaining nor arguing. You are informing.

The more fundamental issue here, though, it seems to me, is curtailing their emotional blackmail as it happens. Even if you were to speak with them as little as let's say once per year, that one time is going to be miserable for you if you allow them untrammeled freedom to berate you. My thought, then, is to work on finding ways to end or severely limit conversations that have become abusive. Don't listen to the endless monologue.