If you have purchased the Ad Removal package for Fongo and you uninstall the app and then reinstall it, you must restore the purchase from the Add-Ons screen to remove ads again.
If the 'Restore' button does not appear in your app you may have disabled 'In-App Purchases'.
To re-enable 'In-App Purchases' you can do through the Restrictions Settings
Go to Settings->General->Restrictions
Then turn on 'In-App Purchases'