Related stories are controlled by the FeedBlitz RSS feed that powers mailings. This differs from many other mailing features controlled by the settings found under your mailing's Edit Campaign page.
To change the Related Stories Feature, you will:
Start at your Site Dashboard and navigate to the RSS feed that serves as the source of your mailing.
From the feed's dashboard, click Settings > Feed Settings
Click the small drop-down arrow to the right of Settings and select Feed Settings
On the next screen, select the Per-Post Customization and Related Stories Tabs.
You can then set the number of related stories to None, which will turn off the option.
Click Save Changes to apply your changes, and Related Stories will no longer appear in your mailings.
If you have any questions, please don't hesitate to let us know. You can find all of our great support resources and how to get in touch with us via email, chat, or phone on our Support Page.