A backup can only be deleted when it has been archived. A backup is being archived automatically when BackHub can’t access the original repository at GitHub any longer. That happens either when access is being revoked for the installation, for this specific repository or if the original repository has been deleted on GitHub. Archived backups are listed in a separate section in the bottom of the page and are indicated by a dashed outline.
You will find the delete button in the footer section of the backup panel. When you delete a backups it’s being removed permanently and can’t be restored any longer.