@pn2 Welcome to forum
You can cancel vouchers using My Account and you won't be charged again
If you cancel card details you won't be able to use demand services, like iPlayer, and more.
@Seanwadham if clicking My Account takes you to the homepage, you've logged in with the wrong username. It's possible to have multiple Sky iDs and only the one attached to NOW TV will work. If you've had Sky Broadband or TV in the past your browser may have auto completed your old username. Try other usernames you might use or the forgotten username link on the login screen.
Also you could look through your emails for communications you've had with NOW TV and see if any of these include the correct one, or load the app which you've been watching on and check the settings to see which username is on that.
Once you've got the right one, clicking My Account will show the passes and you can cancel them with a few clicks.
Just go to My Account ( Top Right) and cancel vouchers - you won't be charged again
If you remove credit card and are using NOWTV box it will cease to work for demand services.