Unsubscribing from SmartSource.com
If you find yourself unable to unsubscribe from SmartSource.com, giving them a call might do the trick. I called the New York office of their parent company, News America, at 1-800-462-0852. The operator kindly transferred me to 212-782-8095, which, though surprised by my call, took care of the issue.
Note: This is only for a last resort. After you have tried the unsubscribe link in the email multiple times. After you have followed their backup instructions to email the webmaster multiple times.