Here's how to cancel a client's billing subscription from your Dashboard.
From the Site Details page, click on "Cancel Subscription" in the Client Billing window.
You will then be prompted to confirm the cancellation and choose whether you want to keep the site online or take it offline when their billing is cancelled. Once you cancel the subscription, your client will no longer be charged.
Please Note: You can always "Reactivate" a cancelled subscription from the Site Details page. If you reactivate a subscription you can choose a new plan and begin charging your client without having to re-enter their credit card information. You can also then put the site back online.