Yeah I meant no centralized instance. I hope that was/became clear.
As I said there was an approach by amethyst. There's a kind 1984 (yeah, exactly) event to report users. There was quite some discussion about it back then. Maybe it could be used in some more advanced way, like giving users more choices who they trust in terms of reports etc. but it's in general the same effect as blocking users but by some automated way.