I suppose that could be abused however.
When you click "Report as spam", if it detects it's a mailing list, it offers to (try to) unsubscribe properly:
http://gmailblog.blogspot.com/2009/07/unsubscribing-made-eas...
It's a bit strange that they don't give you the option then to unsubscribe without marking as spam (you can do it but with more clicks), but I would think that if you unsubscribe and mark as spam the overall system doesn't take the spam factor at all or as much. In that case, it's probably limited to your own account.
I actually do unsubscribe, but I would imagine that many just hit the "spam" button to "make it go away".
Perhaps you should make your unsubscribe links easier to find?
Checking the box in my Gmail inbox and clicking the "Spam" button is much easier than opening the email and hunting down the unsubscribe link.
Personally, I've noticed an increasing affinity for the 'tag as spam' button.
It takes a certain amount of effort to scroll down, and find the unsubscribe (I'm not even sure if it's there), vs. having my inbox cleaned up by just clicking the darn 'tag as spam' button -- dead easy. Also, there's the uncertainty about how daunting the unsubscribe experience might be. These days, quite many startups get their users to go through tedious surveys before they can unsubscribe completely. This results in a sticky association that's hard to undo: 'unsubscribe' is a hassle.
Are there ways around this?
How about a large button above the fold that says 'Don't mail me.', 'Junk!' or similar. Something that indicates they'll have their intended results: less of this sorta mail, and a cleaner inbox. On the other hand, this might result in more unsubscribes.
I'm assuming that the majority of people are not even opening the emails though. Hitting the spam button is just too easy. I think the system is broken.
The comments at the following link show what I'm referring to -- people taking the easy way out: http://ask-leo.com/why_shouldnt_i_use_the_report_spam_or_jun...
It's probably partly our fault: we geeks have been telling our friends and family not to open suspicious or unsolicited e-mails for years.
More often than not, I find following those unsubscribe links very annoying because I have to log in and fill out 5 different forms.. and then it doesn't even work :(
Maybe your emails aren't appreciated in the spirit they are sent. Just because you cajoled someone to check a box to use your site, doesn't mean they appreciate your email campaign.
He didn't "cajole" anyone to do anything and the truth is the vast majority of people don't care one way or the other.