This would save us a lot of money I think, saw this option when setting up an ad today.
For Android, users that already have your app installed are automatically excluded as long as you provide us your app's package name in the app settings page.
For iOS, you have two options. Provide a deep link for your app when creating an ad under "App Exclusion URL". This will allow us to automatically exclude users. Note, that this is not your Apple App Store or Google Play URL, but a deep link for your app.
Alternatively, you can add an array key called URL types to your app's main .plist file, and give it a single array sub-item called URL Schemes. Then add a single item to URL Schemes with your app ID prefixed with "fb", e.g. "fb1234567".