@Anonymous User
Welcome to the forum. Are you sure you're logging in with the right username?
Usually if you keep being asked for payment details when you try to cancel, you've logged in with the wrong username (for instance you might have logged into an old Sky ID you've used with Sky TV or broadband my mistake, or an old NOW TV account you forgot about).
Try logging in with any other accounts you've set up (check the My Account screen of the NOW TV app on any devices you use to watch on to see what your username is, or check any emails you've received to see if they have the correct username on them).
When you're logged in with the right account, you'll be able to go to http://account.nowtv.com/passes to cancel.
If you are still stuck, have a word with live chat who will help you get the pass cancelled. To get there got the following help page and click the start live chat button.
http://help.nowtv.com/contact-us/Cancelled-but-still-being-charged
