Here is my own userscript that removes any matching URL portions from Google search. It does it in a fancy way with stylesheets so that you never see the Quora, etc results even temporarily before they're removed. It also fixes Google Images to link directly to images again, removes all the tracking URL obfuscations, and more.
Should be more robust than the other alternatives listed, but if not, let me know. I'm always up for improving it more. Contributions also welcome.
Blacklists domains from search results in 1-click
Firefox Version https://addons.mozilla.org/en-US/firefox/addon/personal-bloc...
There is now another alternative, ublacklist
https://chrome.google.com/webstore/detail/ublacklist/pncfbmi...
[1] https://chrome.google.com/webstore/detail/unpinterested/gefa...