You do this through the Privacy section in your account settings page.
This can be accessed from the stv.tv site - and logging in (if you aren't already) - then clicking on your name in the top bar, and then the Privacy from the options below, you will then see the option for Email Newsletters in the blue bar below
Alternatively you can use this link to access the page directly. : https://my.stv.tv/privacy/newsletters
From the Email Newsletter section you can manage the options for receiving all emails or you can do this for the individual newsletters.
Please note that it can take up to 48 hours for the change to have an effect.