I keep getting spam email. How to unsubscribe?

I apparently subscribed to some site that offers tours as a reward. I keep getting spam emails about how I won in a ďwin free tourĒ contest. How can I unsubscribe to this kind of email? Iíve tried unsubscribing to the site through the email but I somehow still get these kinds of emails once every week. Itís starting to annoy me. How can I stop this completely?