[hajile]

Why not crowd-source it?

All Google needs is an obvious, one-click "spam" button for logged-in users. Clicking should add a site to the user's spam blacklist (which they should be able to review).

They know about what users search for and how those searches overlap and separate users into groups (not to mention all the individual details they have access to). When sites are marked as spam by enough different types of users, those sites can then be manually reviewed and their content blacklisted by Google preventing the same or very similar sites with a different URL from appearing.

Unfortunately, Google makes a lot of money from these ad/spam sites, so they have a perverse incentive to keep allowing them.

[rightbyte]

It is great effort to make a high ranking spam site and trivial to destroy it for Google. It would be a winning battle.

I think that Google just don't care and are happy to be a search motor for Reddit and Wikipedia and to answer questions like "How old is Lady Gaga?".