Web of Trust.

It's the idea that if two nyms (account/people) mutually follow a third nym, there's a good chance that that third nym is a "legit" and therefore their notes should make it through a filter.

Alternatively, if two nyms block a third, the filter can block the third nym.

I would need way too much time to explain why I don't like either of these types of filtering even if they have pros.