WilyNily..I totally understand what you are saying...my own DD is not a classroom type. But she has almost finished, and as stressful as it is, it is one guy in one class. Drop the class. Yes, kids get it drilled into them about college these days because life experience no longer cuts it for a good job. If this was her first year, I'd say withdraw and take a break..but 4 years?? I hate to see anyone throw away all their hard work for one bump in the road (an annoying bump, but not insurmountable). We run into a lot of Joe's in our work experience. You either have to learn to deal with them or you will always be hiding under a rock. OP, I know you are burnt out (been there, as we all have) but one person in one class should not cause you to walk away from it all.
There is also the fact that it's very difficult to convey the impact of bullying/hostility in work or school in text form. I was undoubtedly bullied by my Former Horrible Boss, but any attempt to explain it to someone who wasn't there makes it sound like I was flipping out over small incidents and being "too sensitive." I wasn't, but even some people who know me well and were even there for some of the incidents thought I was overreacting because they hadn't experienced the overall pattern of behavior, which was, frankly, vicious. Those kinds of reactions just feed into the gaslighting - "See, everyone else thinks you're too sensitive, too! I'm trying to HELP you by insulting you and nitpicking constantly! You need this kind of micromanagement because you're too incompetent to do it on your own." Only, not in those words, as that would be too obvious.
The OP tried to get her problem resolved, and was blown off. She escalated, and was blown off. I wouldn't want to have anything more to do with that institution, either, if they're not willing to take concerns about a hostile environment seriously.
All that being said, OP, I think it would be good for you to formally withdraw, for the reasons mentioned above, as well as being able to officially, in your own mind, say, "Yes, I'm DONE with them." It could be a form of closure. You should be able to do it online or via email, so you shouldn't need to talk to them at all.
P.S. Bullying/hostile work and learning environments in academia has become something of a pet cause for me, given some of my experiences. There are a few resources out there, though not as many as there should be.