You'll want to open your client's dashboard and cancel their subscription from there. Let me guide you how to do it in your QuickBooks Accountant Online (QBAO).
Here's how:
Go to the Clients menu.
Find the client you want to edit the info for.
From the ACTIONS column, select Edit client link.
Under QuickBooks subscription section, select Your Account link. It'll route you to your clients' dashboard.
From there, go to the Gear icon and select Account and Settings.
Go to the Billing & Subscription menu and cancel your clients' subscription from there.