How can I cancel a subscription

You are sure the subscription is on being paid by that account, its payments show on its purchase history : See your purchase history in the App Store or iTunes Store - Apple Support ?

If they do and the subscription isn't showing in : View, change, or cancel your subscriptions - Apple Support

try contacting iTunes Support and see if they can stop it for you : http://www.apple.com/support/itunes/ww or https://www.apple.com/emea/support/itunes/contact.html

You've checked the account's purchase history to confirm it ? Have you contacted iTunes Support ?