The only way I found is to manually subscribe to a thread, then it will ask you if you'd like email updates. If you post directly on a thread it will just send you emails. As far as I know you can only turn it off on a computer
I don't think you can change the notification options from the App. To turn off email notification of subscribed threads, load AA in a normal web browser, and go to "User CP". Selection "edit options" on the left side of the page. Scroll down a bit and find the option for Default Thread Subscription Mode. Change that option to "no email notification".