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.
For lack of better words, Quora has always struck me as expertsexchange 2.0. There is nothing wrong with being the reincarnate-2.0.-site, but Quora seems to be resurrecting demons and zombies and death we'd rather stay in the ground.
(For comparison, I consider Reddit, with its thousands of subreddits to be a form of usenet 2.0)
They've done heaps of annoying shit, but that in particular is one of the few things they haven't done.
If an opt-out takes more than ten seconds (find opt-out, click it, close tab, alt+tab back to email) I add it to my spamfilter. And that means I will never read email from that sender again, not even in my spam folder, unless I am expecting e-mail and (temporarily) remove it from my blacklist.