You have to call them to unsubscribe. It should have said so in the fine print when you subsrcibed, or if you do a search in the help. Actually it is right on the frontpage of the help, question 10,
HERE is the direct link.
It is a very common practice with most companies, especially credit card companies, to make you call them to unsubscribe. That way, they can try to talk you out of unsubscribing or sign you up for something else.
Good Luck.