I found the Firefox extension "Search Filter" [1] to be immensely helpful at combating SEO spam. It allows you to define a list of URL patterns to remove from the search result list altogether, you can use mine as a start for your own if you wish [2].
[1] https://addons.mozilla.org/en-US/firefox/addon/ublacklist/ [2] https://news.ycombinator.com/item?id=29794372