I probably don't understand Mastodon well enough so I might be tooting misinformation. ;)
But to answer your question (and I'm probably oversimplifying a problem here), imagine Mastodon was more like WordPress, and less a bunch of federated servers. I set up a site, just for me, and "opt in" who I want to see—and no one else gets through. If you're worried about someone's content, just oust them/unfollow.
Granted, this kind of federation requires a centralized follow tree which would have to be hosted somewhere else...OR with a different set of federated sites...(getting complex here)...
I still don't really understand what you're suggesting, but the hint I'm getting is that you're suggesting something interesting, but whatever it is, it's really not Mastodon.
Local instances are like Twitter - you toot, everyone on the instance can see what you say. You can follow someone, and their tots turn up in your specific timeline. The flow of control is that you control what you see.
You seem to be suggesting that people should somehow control who can see what they say. In a sense you can semi-do that. You can set all your toots to private, and then confirm "by hand" anyone who wants to follow you. In this way only those you have specifically confirmed can see what you say.
Fair enough, that may be what you want. But in what sense would that ever be "social media"? You may as well have a mailing list.
So with Mastodon you can follow people, and then only ever look at your timeline. That means you'll only ever see things by people you have specifically asked to see. And you can set all your toots to "Private", so only people you have specifically permitted to follow you will see what you say.
But to answer your question (and I'm probably oversimplifying a problem here), imagine Mastodon was more like WordPress, and less a bunch of federated servers. I set up a site, just for me, and "opt in" who I want to see—and no one else gets through. If you're worried about someone's content, just oust them/unfollow.
Granted, this kind of federation requires a centralized follow tree which would have to be hosted somewhere else...OR with a different set of federated sites...(getting complex here)...