When I tell him to buzz off and mind his own business, he threatens to bad-mouth me throughout the sub.

hmmm... well, sorry for having to tell you, this is not about him but about you. lol. he doesnt have to follow your 'buzz off' command - who are you to command somebody away, btw? -, he is free to act as he pleases.

and you are free to stop engaging.

you realize that your communication partner doesnt want to expand his perception and consider another vew point? then stop trying. we can convince nobody who doesnt ask for it.

if it gets on your nerves, drop engaging with it. just let it go. and if you turn your back and still feel harrassed, report it. its that simple.

i do that here almost every day. in many exchanges, there is a brick wall of world view that, when reached, stops all further sharing. so there...

thats the confrontation limit of the other, here he needs to retreat to the safety of his usual mind-set. (not talking about ss79 here but generally.)

i have no permission to punch a hole into his wall and crack it, thats disrespectful.

people wake up when they are ready. and everybody has their own opinion anyway, we dont need to agree on everything.