When you receive the Newsletter email, 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 email address and will remove you from the newsletter distribution list.
Please make sure to enter the same email 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 your Account page (as shown below).
Click Profile to access your account info, then uncheck the box next to Send Me the Newsletter.
Then click on Update to apply the setting.