Just putting "-quora.com" (no quotes) on the end of a search seems to do it just fine.. you could have a long list of them you copy/paste in every time, or I imagine you could write a grease monkey script to do it for you automatically every time you click "search"
Or -site:quora.com if you are OK with pages with links to quora but you just do not want a result from that domain.
Yep, and a quick way to search with/without blacklisted sites included. I'd pay for that.