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 your Account page (as shown below).
Click Profile to access your account info, then un-check the box next to "Send Me the Newsletter"
Then click on "Update" to apply the setting.