It is not possible to delete devices on an account and it is not possible to delete the account when it has printers which has been collected.
An inactive device will only be billed the months it is active.

You can "Hide" an account if it is not active anymore.
If you want to make sure you do not receive data on an account, you have to stop and uninstall the Data Collector at the customer. 

You can delete a user if you want to use the email address again in a new account. Go to Admin --> Users and select the Username and click the Delete button.

You can also deactivate the Data Collector online.Go to Admin --> Networks and select the magnifying glass on the Data Collector and select "Deactivate DC"