Every email sent through SimplyCast includes an automatic unsubscribe link at the bottom. You can choose to unsubscribe from an individual list, or choose to be added to the sender's unsubscribe list to prevent any emails from being sent to your address.
If you feel the message you received was spam, you can use the "Report It" link to register a complaint about that particular sender. This will also unsubscribe you from their future mailings.