When you as an agency end a relationship with a client, you will want to dispose of the sub-account.
Note: This process will remove the account from your inventory.
Step 1: Delete ALL call tracking numbers.
Go to Assets->Call Tracking, click the 'x' by each call tracking number. If this step is not completed, you will not be able to proceed.
Step 2: Close the account
In the account billing section of the account, click the 'Close Account' button
This is a PERMANENT process. You have to assume this process is permanent and that all data in the account will be lost.