Dear X.....

My recent posts have been mostly about setting boundaries and disentangling myself from less-than-helpful people and/or those who have no respect for my boundaries.

Assuming you have people like that in your life, what is it that you would tell them (assuming it would help... in my experience, it's best to keep these thoughts to myself).

I'd say something like this:
Dear X,
You may have noticed that I'm not responding to you as I usually do, and that something has changed. What has changed is that I've decided that there are other things more important than engaging with you. Things like, my own boundaries and my own goals and priorities. You often criticize me, and your criticisms may even have merit, however, you have demonstrated that you don't really care about my well-being, in fact, you use those criticisms as weapons, and your goal is to harm.
I'm in the habit of spending my time with people I respect, and people who help me to be a better person. I'm afraid that if I spend time with you, that I would become more like you, and if I became more like you, then I'd be moving in the wrong direction. You're not my enemy, but you have demonstrated that I'm your enemy. For my part, I wish you well, and I truly hope that you can change and become a better person.
You may change, and/or I may have been wrong about you... if that is the case, then I'm always ready to reassess my feelings and opinions about you, and the place you have in my life.