It takes several clicks, and doesn't always work. If the sender is a spammer, you're opening up a can of worms by exposing yourself as a real person - so you'll get even more spam (has this ever happened to you?)
And if you actually subscribed to this email, you're really hurting the sender by unsubscribing - most email delivery services stronly penalize senders with high unsubscribe rates. You also shouldn't mark those emails as spam - that's just bad karma!
Just drag an email into your @SaneBlackHole folder, and you'll never see an email from that sender again. All future emails will go straight to trash.