Anyone using PubNub?

We have a mobile app and send push notifications to the device for scheduled events and chat notifications. Right now, if you’ve installed and signed up, you receive the notifications, but if you uninstall, we still try to send them. If anyone is using Pubnub, how do you filter out devices that no longer have the app installed? We’ve been asking their sales engineer, but have yet to get a good answer (probably not surprising since it will reduce our usage) TIA!