If you simply want to stop receiving notifications from TestFlight, follow the procedure here.
If you wish to stop beta testing completely - open the TestFlight app, select Delta, and tap on 'Stop Testing' at the bottom. This will remove you from our Early Access Program (or 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.