How can I deactivate a Market Data feed?

A subscription could be deactivated by clicking on it on the Accounts screen. That brings up a modal window:

Once a date is set in the (deactivate) Schedule Deactivation calendar control, the bundle will be listed in red when shown in the list of subscriptions:

IMPORTANT: The deactivation date field specifies the day after which the subscription will be canceled. As a result, once a subscription has been activated, it will be live at least for the current day. That will result in collecting one monthly fee from the user’s account. However, scheduled subscriptions could be deactivated before their effective date which will not trigger a charge.