You can manage how TestFlight can contact you for updates. Open the TestFlight app, select Delta, tap on Notifications and uncheck Mobile and/or Email Notifications.
If you want to completely stop beta testing Delta, you can tap "Stop Testing" in TestFlight, which will remove you from our Early Access Program (beta testers list) and you will no longer receive any new builds or notifications.
PRO TIP: You do not need to worry about this if you are not testing beta Delta builds. You can manage Delta normally from the App Store.
PRO TIP 2: The Early Access program is now closed. You can learn more about it here.