Deleting app data from Game Center
I don't think Apple has a way to delete things or let you delete things - this is an iCloud general feature / design goal.
It's up to each developer to clean things (or ask you if you want things cleaned) when you delete the app or use the code in the app for resetting things.
You'll need to work directly with that vendor unless Apple makes a change going forward.
See:
- Can I delete an iOS game and retain my progress?
- How do I delete data for one game in the iPad Game Center?