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.
The admin https://lemmy.dbzer0.com/u/db0 from the lemmy.dbzer0.com instance possibly made a solution that uses a chain of trust system between instances to whitelist each other and build larger whitelists to contain the spam/bot problem. Instead of constantly blacklisting. For admins and mods maybe take a look at their blog post explaining it in more detail. https://dbzer0.com/blog/overseer-a-fediverse-chain-of-trust/
So defeating the point of Lemmy? Nah, that's a terrible "solution" that will only serve to empower big servers imposing on smaller or even personal one's.
The (simplified) way it works is it reads data from the public observer's API and check if ((total users > (totalPosts + totalComments) > susScore) as a "suspicious" community. "susScore" is configurable if you want to run your own instance of it.