Note: Usage buckets that are configured on services and other entities are in use and cannot be deleted. In order to delete a usage bucket that is in use, first determine which entities are configured with the bucket by checking the References tab and then edit those entities to use an alternate bucket.
Under the Buckets heading on the left select the usage bucket you wish to delete
Under the same Buckets heading on the left click the icon
On the confirmation dialog that appears select Yes