Swipe a notification to the side a little bit, and you get a gear icon. Tap that, and you can set that app's max allowed notification importance. You can block all notifications, relegate them to only being visible when you pull down the notification bar, or allow normal icons but not "peeking" notifications.
Yep, there's a new feature in Android O that gives you full control over notifications. You can have them make a sound and pop up, only make a sound, make no sound, or not show up at all. And you can set that per app, and even per notification type if they are targetting O api
