Boundaries- do I need to tell the coworker?

I have decided to no longer read emails addressed to me from a coworker. They copy in my boss and colleagues and things are escalating to the point of demonstrably untrue allegations and ad hominem attacks. I’m not coping mentally as I’ve had a model for him where he should behave honestly and decently in response to my professionalism.

I’ve tried with thought work not to care/give him power but after 4 months I’ve decided not to continue to argue with reality and with someone who is fabricating complaints about me. He is not engaging with work in these emails, rather, he is criticizing my performance. My question is, do I need to tell him I am no longer reading/replying? Am I still empowered if I choose to simply privately institute this boundary? Is it fair to put in a boundary when the other person doesn’t know about it?

I’ve effectively changed the locks to the penthouse in my head, where if given him free rent. I believe that telling him will escalate things further and I am intimidated by him, or at least, haven’t reached a neutral place in my thought work. I do not want any more engagement with him beyond matters that are neutral and strictly work related.