Often spam emails will contain an "unsubscribe" link or button.
Unfortunately, if the email is really spam and not something you explicitly signed up for, using this "unsubscribe" link is in fact likely to increase the amount of spam you receive.
Anybody who tries to unsubscribe in this way is just letting the spammer
know that his or her email address is valid. The spammer can then confidently send you even more spam, with the happy knowledge that your account is active and that you actually read your email. If someone is unscrupulous enough to send you unsolicited advertising material in the first place, it is quite unlikely that he/she/it will honor a request to stop sending it.