Use Show Cancel button on profile on the Order: Subscription tab:

• Client can Cancel Subscription - a Cancel button on the customer's subscription page cancels immediately.
• Client has to Request to Cancel Subscription (default) - the button becomes a Request that notifies you; you then cancel from admin.
• Client cannot Cancel Subscription - no cancel option is shown.

This global choice is combined with the per-subscription Cancellation mode set by an admin on the subscription itself: the more restrictive of the two wins, so you can lock down a single customer even when the global setting is permissive.