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.
In other words, for every server that does not adhere to the other rules mentioned (captchas, email requirements, etc), automatically de-federate from all of them.
If you're not familiar with what "de-federate" means, check out this article on federation concepts