Subiscriptions are subject to the Pixologic store Terms & Conditions. These rules are explicitly agreed to as part of the process of purchasing a subscription.
You may cancel your subscription at any time prior to the end of the current billing cycle. Once canceled your subscription will not rebill and ZBrush will then stop working on the rebill date.
A subscription can only be canceled via the My Licenses page. Once logged in under the email address that your license is associated with you will see your subscription. It will be flagged as ACTIVE. Click the button to manage your payment/billing information. You will then see "Cancel Subscription Auto-Renewal" button as shown below. Click that button.
After your subscription has been canceled successfully, the My Licenses page will change its flag to state that it is a CANCELED subscription. That way you will know that you will not be rebilled at the end of the current billing cycle.
If your serial number shows that your subscription is ACTIVE it will be rebilled! Please note that all subscriptions rebill at 12:01 am on the rebill date. If you wish to cancel, be sure to do so before the billing date arrives.
Deactivating ZBrush is not the same as canceling your subscription. The purpose of the deactivation system is to enable you to move ZBrush between computers. If it canceled your subscription then any time you needed to change machines you would have to start a new subscription. That also means that you would forfeit any time remaining on the current subscription. This would be both overly complicated and unfair.
Pixologic cannot issue refunds for rebilling of an active subscription. This includes failure to cancel prior to the rebill taking place or mistakenly thinking that deactivation would cancel the subscription.