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.
Agreed, this is actually the use case for a blockchain. Intances should produce documents attesting to the quality of other instances. This would allow each instance to construct white or black lists off thes data and propogate info on bad actors or new quality servers. It would be decintalized, consistent and signed.
There is a number of blockchain attempts to do this already. Because they are servers, it should work well here.
https://dspace.mit.edu/bitstream/handle/1721.1/130295/11280_2021_Article_869.pdf?sequence=1&isAllowed=y