"unsubscribe" links vary in position, language and visibility in various clients. Making something beyond "this is spam" part of most mail clients, perhaps with reporting back to the originating sender, would help.
So many don't dare to use such links and rather click on spam.
The only solution could be some "trusted" functionality that goes via the own mail provider of the receiver. But of course the mail provider can not simply send information to the sender of the eMail .... So the thing gets complicated. As much I learned, for spam clicks there is something like that available ... some kind of trusted feedback chain that gives information to trusted senders, that some mails where labeled as spam. Thus those senders can (indirectly) adopt their eMail campaigns.
That's only true of the dubious "viagra" style spam, where they got your name from a list. I don't think those even bother with "unsubscribe" links any more. I only see unsubscribe links from places where I've had to give my email up to buy something or sign up to a site. Those are generally legit and most of the techy/startup web sites will unsubscribe you immediately.
The next tier are the sites that unsubscribe you but take more than a week and will keep spamming their dumb newsletter in the meantime.
The final ones are either broken by stupidity (it's amazing how many web developers cannot grasp that "+" is a legit email character), or willfulness and will keep spamming no matter what. I block these at the SMTP level with 503 messages (usually containing some personal insults and swearing) as soon as they "RCPT TO" the unique email address I gave them.
http://gmailblog.blogspot.com/2009/07/unsubscribing-made-eas...