When you cancel a 7-day trial, what you are really cancelling is the continuation of the service after the 7 days, so it remains active for the rest of the 7 days.
You first attempt at cancelling must have failed, or you would not have been able to make a second attempt.
And that must have failed also, or you would not have been charged for the coming month.
Cancelling involves about four steps, and it’s too easy to bail out before it is finished, thinking you have cancelled when you haven’t; your best bet is to look again at your My Account/ Passes & Vouchers page, to double-check that it shows no upcoming payment date.
However, it’s a bit late for that now. Except that the meter is probably still running, so try the Cancel again, and keep at it until the P&V page agrees that you have cancelled.
And if you have not taken advantage of the service during the month you accidentally bought, get in touch with NowTV on Live Chat and explain the mistake; they can check you have made no access, and you should get a refund.
NowTV don’t make getting in touch with them a whole lot easier than cancelling, but either of these two links should get you there:-
Get in Touch (all the options, and the best place to start):-