What is the tell between a spam site from an authority?
Spam: thin site, lacks original content, few pages indexed, crawled infrequently, ranks for a small selection of keywords, weak backlink profile
Authority: many pages indexed, thick site, loads of original content, crawled frequently, ranks for a variety of keywords, diversified backlink profile
Page Rank used to be a strong indicator of this, but currently a better indicator is how often your site is crawled. That's why one authority link can do worlds more for you than hundreds (or thousands) of spammy/low quality links.
Not everybody blogs about everything going on in his/her life.
I would expect that these true spam sites exist as a link network somewhat seperate from the rest of the web. You could start manually defining filters based on what sites are actively selling links on the digital point forums.
We may have actually had this conversation in the real world, come to think of it.
* i would assume that the problem parallels with spam emails. i think webpages offer more clues to what the page is about than emails though.
Spam passess a single toll gate on the way to each user. You have an address book with contacts and previous conversations and a whole pile of data about previous 'known good' emails in the inbox.
Spam filtering has gotten pretty good. A lot better than google at filtering out 'bad' search results.