Frank The Tank so I can come into your house whenever I want and do whatever I want, cool, I've always wanted to see how my rifle works on a couch
Thankfully that is not the way the rest of the world works. The law does not recognize "social media" or any kind of social contract (facebook bans people ALL THE TIME for tons of reasons). A website is not considered a public place. There have been case rulings that deal specifically with this (some guy tried to sue a site for banning him) and the courts ruled that server owners can do whatever they want and that users do not have any inalienable rights to use somebody else's hardware and software over the internet. Yes that is what you are doing by going to a site, using someone else's computer ( this is why all the hacking laws are prosecutable, you can't just come and use my stuff with out my permission). This is the basis for all Internet hosting and commerce. With out it the entire system would crumble and be useless, since it means that I have rights to install whatever I want on your personal computer since it is broadcasting packets over a public internet (the ones you used to post here).
Is it Anarchy? Sure for some definitions of the term. There is nothing preventing me from doing whatever I want on my systems, and the same goes for you. But what you can't do is go and apply your rules to my systems, there is a concept of ownership, so that aspect is not similar to your traditional anarchy. Your traditional anarchy degenerates in a bullying "might makes right" system and that is not what we have here.
It is more similar to your your modern "free" society. "Do whatever you want with your own stuff, just don't affect other people's rights to do the same thing"
I am not BZ, technically Tim is (he owns it), and yes BZ is somewhat pointless with out a community. That's why everyone is nice enough to let people use the servers by default with out asking. But that does not mean that they can not set rules, or they are not owned. It just means they allow access by default and deny as needed.
Sig was NEVER banning people just for belonging to a group, he was banning them for breaking his posted rules. Your UN guidelines (not that they EVEN apply here) allow for a cafe to ask a specific person to leave, regardless of race, creed, color, etc.. "We reserve the right to refuse service to anyone" recognize that sign? Sig hag posted rules, people were caught in his rule plugin, he banned them. He didn't want the distraction in chat for his games. Heck he could configure the plugin to make every person say "I am a teapot" and that's be fine, it'd just be a feature of the software you have chosen to use when you connect to his machine. His current featureset includes "connection will be terminated when the characters G and U are sent over TCP", why is it his fault if you don't find that useful? Use some other software that you configure if you don't like it.
That is his right to do on his machine, just as it is my right to say "I won't allow anyone with pink hair into my house", that is my right, and NO law like that can be used for you to force entry into my home.
If the laws didn't allow for servers to decide who has access, I'd shut down every single one of the bzflag public services and the entire project in a heartbeat, because then some spammer could force his way to stay on the forums just by saying he's Jewish. These laws and rules are here to protect that.
The fact that you feel entitled to use his machine as you see fit is the scary part.