I use uBlacklist to get rid of Expert Sexchange and similar low-quality sites in search results, and it seems to work well enough.

An "advanced mode" supporting literal keywords (with and without stemming) and boolean operators wouldn't cost the search companies anything. I think supporting regexp search would be hard: do you search your index for fixed substrings and expand around them? I'm not a search person...

I don't think you'd need much in the way of machine learning to filter out the spam. There are relatively few third-party display ad servers and affiliate networks, and those are the main lazy ways to make money. There's no need to filter out all commercial content; just getting rid of the "passive income" bros would be enough.