we already see that in action with the american right wing communities, don't we? I just hope the biggest fediverse manages to stay diverse, monocultures are no bueno
It's not really a question of whether they can or not. They already do. Sometimes, though, they have opinions that conflict with each other and they defederate each other.
But extreme right-wing communities are already a thing in Mastodon and the most extreme ones tend to get defederated (the worst offender being poa.st), and among the most defederated, they often federate with each other (you will notice that poa.st still has a "vibrant community" of instances that are ok with being offensive).
Yeah, it's like the Nazi bar idea: that if you have a bar and there's one Nazi, you need to kick them out and show zero tolerance, otherwise they'll bring their Nazi friends and your regular patrons will leave and before long, you'll be running a Nazi bar.
The thing is, kicking the Nazi out protects your space and your community, but it doesn't stop them from finding another bar. There's always a Nazi bar somewhere.
I myself occasionally browse the "nazi bars" of the fediverse to get an idea of what the hell they're complaining about now. Similarly I go to the tankie places to get acquainted with which genocide they're denying today hahah, it's a fun hobby to do occasionally, too much and it gets irritating though.
Instances can choose to federate specifically with a group of other instances already. If I wanted to turn mine into a right-wing circlejerk or off the rails conspiracy group, all I have to do is find some other Lemmy instances and add them to the Allowlist, which tells my instance to only connect to those I've listed.
An instance can both make a black list and an allow list. If there are any instances on the allow list, only those instances are federated. So yeah, that is possible.
Email servers work the same way. So far there's still one big email "community", while spamserver.ng is just pretty universally blocked. I think that's strong evidence the strategy works.
Now, a much more relevant question is how do you run your instance.
Well, a huge caveat to that is that there are world class Researchers who create constantly adapting intelligent spam filters to keep spam out of inboxes. Maybe the fediverse will have something like that someday! Who knows!
I think your underestimating how real the battle against spam still is in the email space. If you use Gmail you've got a huge company doing the work for you. If you work in IT and run a corporate mail server you'll see that we pay hundreds of thousands of dollars for anti-spam/phishing filtering.
Bot spam is totally separate and unfortunately I don't see how you'd be able to port the work over.
So how would one get defederated?
Say someone creates an instance for questionable material, does every other instance have to manually add it to a blocklist?