Let's start by assuming that you use your email service's SPAM filter to automatically send unwanted em-mails to the SPAM folder, which, in case of Gmail, are delted automatically after 30 days. Alternately, you can use Gmail's "Skip the inbox" function to divert certain email to a specific folder. What's the other thing you can do?

Never, ever click on the "unsubscribe" at the bottom of a marketing email. If you do that, spammers will know that yours is an active email account and they will spam you more! Tagging these emails as SPAM is enough.