Can Android 6 backups be manually deleted? When using the new backup service introduced with Android 6, is there any way to explicitly delete an existing backup of an Android device stored in a Google account?
Google Operating System blog (unofficial) says


  The Android section from Google Dashboard shows a lot of useful information about your Android devices (IMEI number, registered date, last activity date, carrier) and it also lets you delete backup data. "Please note that new backup data will be created if backup is enabled on any of your Android devices," informs Google.


XDA post-  How do I delete Google's Cloud Backups? says the same, with a picture below



Disabling backup from Settings will prevent future backups but not delete earlier ones

Disclaimer: Unverified, since I don't make Google back ups