Please. Captcha by default. Email domain filters. Auto-block federation from servers that don't respect. By default. Urgent.
And yes, to refute some comments, this publication is being upvoted by bots. A single computer was needed, not "thousands of dollars" spent.
Because you shouldn't. Just like with other federated systems like e-mail or OAuth/OpenID, you don't create accounts everywhere, you use ONE account everywhere instead.
You should. Imagine e-mail server admins would start banning other e-mail services based on political or religious views and whatnot, that would fragment e-mail system and eventually destroy it. The only reason to defed other instance if such instance breaks the law. Just like we blacklist fraudulent mail server. Not because we don't like fraud, but because it's illegal.