There are few things more irritating than spam. The good news is that if you contact a company and request that they stop sending you unsolicited emails, they must comply. The bad news, however, is that most companies sending out spam make it incredibly difficult to locate their proper contact information or email address. The Abuse Lookup tool at ViewDNS easily finds the contact information for you.
Just enter the domain name (the web address after the @ from an offending email) and ViewDNS will provide you with the proper email address for you to contact to request removal.