@Anonymous User
Welcome to the forum.
You can cancel your pass by going to your My Account > My Passes page http://account.nowtv.com/passes
You should see a list of any active passes you have on your account, with a cancel button next to them. Just click cancel and follow the steps all the way to the end.
You'll still be able to watch your pass after you've cancelled until the expiry date.
If you have more than one type of pass you need to cancel each separately.
To prevent the possibility of anyone in the household accidentally starting a pass again I'd recommend enabling your payment PIN in your My Account > Payment PIN page
http://account.nowtv.com/payment-pin
Alternately if you are sure you won't be using NOW TV you can pop onto live chat and ask them to remove your card details to deactivate your account. To get to live chat, go to the following help page
and click the Start Live Chat button.
http://help.nowtv.com/contact-us/How-to-cancel-NOW-TV
