When you say basic which version of the now TV box do you mean. The older ones as I remember the black square one did not have a pairing function. You just pointed it at the box after inserting the batteries and it worked?
I’m not sure why whatever you typed in the Search box on the Help here didn’t get you to where you wanted to be (Pair Remote only brings up instructions for the Smart Remote, and I think you have the older one) but this is what I found:-
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.