Welcome to the forum. First off make sure you've cancelled any passes you don't want as they will roll over each month until cancelled.
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 should receive an email confirmation but you might want to take a screenshot of the cancelled pass as well.
If you have more than one type of pass you need to cancel each separately.
As for a refund, if you haven't already your first step should be to 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