That depends on what type of membership you’ve got.
As long as you’ve got an active monthly membership, your NOW Boost will renew automatically each month unless you cancel it. You can cancel at any time in exactly the same way as you’d cancel your membership, by going to NOW Membership.
If you’ve only got a Sports Day Membership on your account, your Boost will only run for one month, and won’t auto renew – so you don’t need to remember to cancel it.
If you cancel your last membership card, we’ll automatically cancel your Boost for you as well.
[EDIT] The link I gave doesn’t quite go to where I picked it up from 😢
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.