I think the average user doesn't know what it means to flag something as spam. Indeed, I didn't before I read this post.
To the average user, I would guess that flagging as spam means "Don't show me any more e-mails from this address." I think few would guess that it would have any negative repercussions for the sender, or indeed that the sender would ever find out about it.
I flag anything as spam which meets one of the following criterion:
1. unrequested
2. no email opt out (looking at you AppSumo) and not accepting mailinator
3. requires more than clicking an unsub link and perhaps an extra button on the loading page. I will not enable JS to do so.
I have marked several mailing lists, like Postgresqls, as spam. I tried to unsubscribe from that particular one for 10 minutes then gave up.
If you are sending emails that people don't want, even if they asked for them in the first place, you may as well be spam. I hope my actions eventually lead to admins being more selective with emailing.
To the average user, I would guess that flagging as spam means "Don't show me any more e-mails from this address." I think few would guess that it would have any negative repercussions for the sender, or indeed that the sender would ever find out about it.