Quora is dangerously close to being a nuisance with all the pointless emails they send. I think I browsed the site a half dozen times max, and contributed to two-three questions. Now I get daily emails about distant friends activity on Quora. LinkedIn this the only site I've been tricked into joining that is worse with their spammy emails.
That said, I usually go with the email-alias trick. Not all websites like emails with a + in their name, so I have a postmap file on my server to map service@domain to my real address. If I want rid of that sites email because they won't respect my unsubscribes, I just remove that alias.
I've also heard of people using the likes of maildrop to /dev/null email they don't want. I have maildrop set up, but no rules in force to delete stuff, just to move.