Cancel is in My Account/Passes & Vouchers, and in this case would be under your Mobile Sports Pass.
If it doesn’t show a Cancel option there, then you have the sort of Pass that does not auto-renew, and therefore you don’t need to Cancel it.
My first reply post in this thread tells you how to access Live Chat, but there is also an option you can reach from that link to email NowTV.
It is is worth putting everything you want to say in an email, and give NowTV time to reply, which they promise they will.
But if they don’t, at least when you next go on Live Chat, you can refer them to the email, with its full description of the issue and what steps you have tried to fix it, and then go from there with them.
Could be a number of reasons, see if there is anything in this linked help article to point you in the right direction.
Also when you cancelled the pass, did you go through all the steps on screen (approx four steps) where a green thumbs up cancelled confirmation message appeared on your screen.
Usually after a pass is cancelled, when you go back to My Account > Passes & Vouchers whilst logged in on here it should show cancelled next to the pass and you should get an email confirmation from NowTV that you have cancelled a pass on your account.
Would also recommend adding a payment pin to your account by going to My Account > Settings & Pins to avoid anybody else who has access to your devices (for example kids in the family) accidentally buying a pass without your permission or authorisation.