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 should receive an email confirmation but you might want to take a screenshot of the cancelled pass as well.
As for a refund, you can try talking to live chat, explaining the situation and seeing if they can do anything for you. To get there go to the following help page
I would also recommend enabling your Payment PIN in your account, this will prevent accidental or unauthorised payments in the future. If you have a NOW TV Box it's easy for people to accidentally purchase a pass. To turn your Payment PIN on, go to your My Account > Payment PIN page