@adnan Only the ones admins haven't realised are undesirable yet. If it's within the ecosystem, the mechanisms admins have for blocking instances should work. If it's web-public, nothing will work.
Personally I've been thinking recently about a conceptual social networking platform (perhaps based around AP and interoperable with this) that allows users to limit visibility of their content to within certain degrees of separation. But that's an incomplete idea I don't yet have time to implement.