When you receive the Newsletter e-mail, there is a button at the bottom to opt out of the Smilebox newsletter. If you click that it will prompt you to enter your e-mail address and will remove you from the newsletter distribution list. Please make sure to enter the same e-mail address you used when you signed up for the newsletter.
You can also go to the "My Account" screen at http://my.smilebox.com
Enter your Smilebox account email address and password. That will take you to a screen with account information (as shown below). On the first screen, click "Edit my account information or password" link.
Then un-check as shown below and click on "Submit" button.