You have evidence of the payments, but do you have evidence of the cancellation?
If it was a good Cancel, you will have had an email confirming this; and if you can supply that to NowTV, then it, together with the evidence of subsequent payments, will convince them that an administrative error has been made.
However, it is rather easy to think you have Cancelled, when you have said Yes three times but bailed out before the fourth and final one; NowTV really should make this less ambiguous for customers.
But even in this case, when NowTV can see that the Pass(es) subsequent to a failed Cancel haven’t been used, a refund is normally forthcoming on request to Live Chat.
So it shouldn’t have to get adversarial, unless you and NowTV are in dispute over the facts of the matter; have you tried talking to them?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.