I’m not clear whether the old email address ties up with an old account, or what.
Nor whether you set this up, or someone else did.
But you can (and usually have to) cancel the Boost for yourself by logging on to the account via Now in a browser, on a phone tablet or computer, and cancelling it there.
But you will still end up paying for a month of it.
If you should have set it up on another account, though, try getting in touch with Live Chat, and perhaps they can move it over for you.
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.