I'll guide you through cancelling your client's subscription. Please follow these steps:
In your QuickBooks Online Accountant, go to Settings ⚙ and then Subscriptions and billing.
Select the Your subscriptions tab.
Look for your client.
From the Select action ▼ dropdown, select Cancel QuickBooks Online.
On the other hand, if you only wish to remove your client from your wholesale billing plan and transfer the billing to them, you can follow these steps: