Nothing, fedi is extremely anarchic by design. If you don't like a server you can move to a different one, or start your own. The big Mastodon servers put in a great deal of effort to police what their users can see and interact with (see FediBlock), but it's impossible to shut certain people or groups out of the network entirely. It's a double-edged sword, but it's a net good for the kind of network it was intended to be, which is a hacker's network; it wasn't designed for normies.