The only way to delete games from your Game Center is to delete the actual game from your iPad. Locate the game in question on your iPad's home screen. Hold down on it until all the icons start wiggling. Tap the little X next to the game. Tap "delete" when prompted. Press the home button to stop all the wiggling.
If you ever want to play the game again, you can re-download it from the App Store at no cost. Tap “Purchases” in the App Store. Tap “Not on This iPad”. Locate the game in question, and click the little iCloud button next to it to download it again. It will automatically be re-added to your Game Center.