You are at fault. If the Adventurer's League has anything to say in the matter, they should not let you DM this guy. You called him stupid and insulted him because "dumb people are your pet peeve." I'm surprised the AL doesn't vet people like yourself. A DM that prejudges people based on his idea of what constitutes an intelligent comment shouldn't be given power over groups of people playing in organized play.
I judge people on their actions. I'm allowed to have opinions. I'm not a robot. I don't like him. So what? I don't let my personal opinions color my judgement. Which is why I kept DMing for him despite my opinion of him. I would have preferred not to play with him but he didn't break any rules and the RPGA is all about giving everyone a chance to play. So, I let everyone play regardless of how I personally feel about them. I based my DMing decisions entirely on the merit of the ideas and not the person suggesting them. And yes, I blew off a little steam by complaining to my friends over my frustrations. My friends would agree with those frustrations, so they weren't only mine. But my roommate who was still DMing at the time would say "i understand, but what are you going to do, kick him out simply because he is annoying? We have 3 or 4 people who show up every week who are annoying. He's not the only one. Since you can't kick him out, learn to let it go."
I admit, I have a harder time letting it go than he does.
Still, I think the fact that I've put up with him for this many years because I believe everyone has the right to play DnD is precisely why I should be the one in charge around here.
This situation sounds irreparable. You should avoid each other if neither of you wants to leave. The player has every reason to believe you are screwing him over given what you've said about him. He can only trust your magnanimity as to why you won't. I would not trust you in the same situation. You have no reason to change your opinion of the man. You seem to want to antagonize him or at least dominate this social situation. It is an unhealthy, mutually antagonistic relationship that most likely will not be resolvable save by mutual avoidance.
To be fair, when that event happened all those years ago, I expected him never to come back. I expected to be yelled at, called every sort of bad name in the book and to have him decide never to play DnD again. But he showed up next week like nothing had happened. It was never brought up again. I began to wonder if his gf had actually told him or just told me that.
But I have no desire to dominate this social situation. I just want him to show up and enjoy playing Dnd. He seems to enjoy it. Honestly, if it's my DMing style, my personality, or even my face that's causing him to get angry, I'm happy to step down and let someone else DM if that solves the situation. No I one else wanted to spent the time preparing an adventure a week to run. It looks like this situation has caused his gf to volunteer to run more games to make him feel better. If that solves this problem, I'm all for it.