Various sites like BGR have written articles on How to remove data from Google's backup and restore cloud.
But it's not possible once you've lost access to the device. Hence your data will remain with Google FOREVER.
It looks like Android's Backup and Restore option makes it impossible to delete data stored on once the user no longer has access to the device.
According to the Answer in:
Backups are stored in sets:
Example: [Device serial][ Google Email] : [3f0e5c90a412cca7] [ email@example.com]
and "You can't make a device start writing into a different set, nor can you wipe a whole set."
Since this is the case, once you lose access to your device, even with your email logged in from a different device, since you no longer have access to the device with the right serial, you no longer have the ability to delete the set from the Backup & Restore menu.
I checked to see if unchecking the "Back up my data" from one device would also uncheck the option across all devices using the same account for backup.
Apparently, this option in the other devices remained checked suggesting that their backup sets remained intact and continue to backup.
Is there a way to delete the backup data set once the device is inaccessible to the user?