If I did not explicitly subscribe to a newsletter, I will hit the SPAM button. I do not care if I completed a transaction with the sender, spam is spam. I don't bother with the unsubscribe links.
I do both. Most companies respect unsubscribe requests and those that persist get blacklisted. This seems like an an easy problem to solve; not sure that it warrants a 650 word blog post.
I'll usually try the unsubscribe button first, if I recognize the name of the sender. But if it is difficult to unsubscribe, or doesn't work effectively, then the SPAM button it is.
I find that most of these "newsletters" stop arriving in my inbox after I do that (with my gmail account).