They get one email and click the Unsubscribe link and think they are stopping all emails. But they don't - just that thread.
There are literally 5 or 6 places that control whether or not someone gets an email on the Account pages, and most of those just control "from that point forward".
For someone that just isn't involved anymore that wants to discontinue getting any messages, they have to unwatch all threads, unwatch all forums, and uncheck the Receive Site Mailings box. That will accomplish what they want but I get people all the time that just don't understand this and want me to "delete their account". We don't delete accounts with public content and people are just too short-spanned on attention to handle this on their own apparently.
Besides, for compliance, doesn't the "unsubscribe" button have to actually fully unsubscribe you from everything and essentially disable all emails?
What the Unsubscribe button needs to do, I think, is cause you to unwatch all threads and forums and uncheck site mailings.
But per the title, the other option would simply be a master "on/off" email toggle. If someone just wanted to stop the madness, one step does it. Uncheck "send me emails" and they no longer get any emails, at all, for anything.