Deleting a backup set does not necessarily delete the data usage from our servers. Some backup set types will ask you if you want to delete the backup data when you delete the backup set.
If you do not see this option when deleting a backup set you will need to delete the backup data manually. This can be done by going to the Delete tab and clicking on the Delete Data button. Alternatively, if you are trying to remove all the data belonging to a computer account, you can delete the entire computer account to accomplish this.