Usually, when a card is lost, people request a replacement card. That card will automatically have their subscription on it, so they can keep using the subscription.

You could block your card and contact customer service directly to cancel your subscription, though. See this page for information about blocking the card (don’t request a replacement card in the process). After you've blocked the card, contacting customer service to cancel the subsription can be done by phone; they are available 24/7 at 030-751 51 55. You may also try to contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Make sure you tell them that you don't have the card anymore.