1. If I'm highly confident it's spam, mark it as spam. (Very rare necessity singe gmail catches spam pretty well).
2. Otherwise it's something I have subscribed for, perhaps without noticing (opt-in by default from some service I have used). Look for the Unsubscribe link, click it.
2.1. If the Unsubscribe link IMMEDIATELY unsubscribes me (landing in a page that just confirms I'm already unsubscribed), ok, then just delete the email.
2.2. But if ANY additional interaction is required--choosing a reason for unsubscription, having to click another button--after finishing this, I will also punish the sender by going back to gmail and mark the email as spam.