You can stop a GCE instance from adding accounts by guaranteeing the google-account-manager service doesn't run.
On CoreOS you can stop this service with
sudo systemctl stop google-accounts-manager.service
and then disable it permanantly with
sudo systemctl disable google-accounts-manager.service.
You may need to create an image with the service pre-disabled (or even delete the unit file) so after you create instances it does not create accounts before you disable the service.