If the subscription you have is a NS Flex subscription, then you can change it to NS Flex Basis in your Mijn NS account in order to cancel this. Changes within NS Flex don't need to be picked up at a ticket machine and you can use the card later should you return. 

If it is a subscription on credit and you are unable to come to the Netherlands, you'll have to contact customer service and ask them to block your card completely. As cancelling a subscription normally requires you to remove it from the card at a ticket machine. However your card will become unusable if you do choose this option.

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