When you once bought the AdFree- In-App purchase there are certain situations where you might need to reload (at no additional charge) the In-App Purchase from the AppStore:
– when you reinstall the app after deleting it
– when you use the app on another device (Apple allows up to 5 copies on different devices)
You will find a button name “more…” after double clicking on the screen. In the next screen you’ll find a button named ‘restore in-app purchase’. Klick on it and (eventually after entering the app store password again) the ads will be removed again.