The save file for this game is located at "C:\Users[REDACTED]\AppData\LocalLow\Clay Dragon Games". You can just delete the "Marble Mage" folder to completely delete your save file, though you should probably back up your save if you want to keep your save afterwards. I do it by having a backup folder in the same path as my save so I can easily copy the folder over again.

EDIT: If that isn't working for you, you might have to uncheck cloud saving in the games settings.