To be in control of the email lists you are added to on Substack, turn on email confirmations.
1. Log into your account on Substack's homepage under Login or directly at this link.
2. Click on your profile icon at the top right and select Account Settings in the dropdown menu.
3. Scroll to the bottom of the page to Advanced Settings.
4. Check the box next to "Send me a confirmation email before my email address can be signed up to any newsletter".