Deleting a game slot directly in-game will not work because the progress will sync back if the cloud save is enabled. This is because your cloud save data is stored in your Google Play Games account. So, in order to really delete your progress, you must go to the Google Play games settings. It should only take a few minutes:
- Open the Google Play Games app
- At the top, tap thenSettings
- Under Your Data select Delete Play Games Account and Data
- Under Delete individual game data tap Delete
- Remember this action deletes all data for this game from all connected devices
- A message pops-up asking you to confirm the deletion, tap Permanently delete
Have fun playing all over again!