Site administrators can manage subscriptions of particular users in Site Manager -> Administrator -> Users. If you choose to Edit () a user and switch to the Notifications tab, you will see all notifications that the current user is subscribed to. You can unsubscribe the user from a notification by clicking the Delete () icon next to it.
CMS Desk users can manage their own subscriptions the same way in CMS Desk -> My Desk -> My profile -> Notifications.
Site visitors registered to the site can manage their subscriptions the same way using the My account web part, on its Notifications tab.