You have to call the customerservice. Removing a card is not the same as cancelling a subscription.

To STOP a running subscription you have to go back to re-install, open mijnNS, and STOP or change hat subscription. IF you have FLEX then change to Basic, if you still hold an old-style ´on saldo´ subscription, then you must ALSO put this new term on the card (ophalen bestelling). Only once you have done all that the due payments will also stop-and with FLEX even reimbursment of any days paid for and not used.

As could be expected all this is also pretty clearly explained in the general site of NS, also in /en/