When you delete an app from your iPhone's home screen, you also erase any data that it may have accumulated. This data includes the app's icon, code, settings, documents, scores. However, if you synced your iPhone in iTunes just before deleting the app, the deleted app and its data remain ever-present in your iTunes library. So, to permanently delete app data and prevent it from re-emerging on your iPhone, delete the app from your iTunes library also. Then you can rest assured that the app and its data are not only gone, but also forgotten.
Tap and hold any app on your iPhone's home screen until all apps wiggle.
Locate the app whose data you would like to delete.
Tap on the quivering app's "X" badge. This deletes the app and its data from your iPhone.
Launch iTunes on your computer.
Tap "Apps," under "Library," on your screen's left sidebar.
Locate the app whose data you'd like to remove in the app gallery displayed on the main pane to the right. Click on the app to highlight it with a thick blue border.
Click on your "Backspace" or "Delete" key to erase the app from your apps library. Next, click "Remove" in the pop-up dialogue to confirm your intention.
Select "Move to Trash (or Recycle Bin)" in the next pop-up dialogue. iTunes erases your app permanently. It will not preserve the app's files and data in your system, nor will it copy the app to your iPhone in future syncs.
For a purchased app, consider keeping it intact in your App library and removing it only from your sync settings so you can reinstall the app with a mere click in the future. If an app manages important data or files that aren't available anywhere else, make a backup of the data before you delete the app.