If you no longer wish to be billed for your client's company, you can simply remove the client's company from your wholesale billing account in QuickBooks Online Accountant.
Once the company has been removed, you will no longer be charged. However, your client will be responsible for their own subscription. They will need to ensure that they have entered their own up-to-date payment information for their company, or they will not be able to renew their subscription the next time it bills.
For further information and necessary steps, see Adding or removing companies from wholesale billing.