From Sports, go to Search (the magnifying glass) top right and enter Darts. The list can be scrolled down a lot further than the four entries shown here, which is as it first appears:-
Or is nothing in this list the Darts you seek?
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.