If marking as spam or unsubscribing are approximately equal effort, I will unsubscribe; if unsubscribing is even slightly more difficult than marking as spam, I mark as spam with no regrets. Even if you don't force me to login but make me do some monkey trick like type "CANCEL" into a box => spam! One click is the only acceptable method, and don't hide the link in a bunch of small print legalese, because I'm not going to expend much effort looking for it while the nice, inviting "Mark as Spam" button is just sitting there waiting for me to click it.
Spam is in the eye of the recipient. If you send me marketing emails that I do not want, it is spam. Even if someone else would want it.
If people didn't mark stuff as spam, then there would be absolutely no reason for marketing people not to send spam to everyone. Even if only one person in 10,000 wants to read it, why not? But the option of marking stuff as spam is a way of making my attention as not free for you.
I don't have the link anymore because this was already a couple of years ago, was someone taking this ages-old advice to the test. Clicking the "unsubscribe" link on all spam delivered to an old spam-infested email address, he found that it reduced the amount of spam received to that address by a significant amount.
Then the domain block goes into place. /dev/null, auto-trash, or whatever the local filter rules support.