See the sticky topic “How to cancel your subscription from abroad”

Officially? You get back here to do what you should have done before you left.

As told by NS? You reach out to customer service which will probably be lenient and help you out.

Practically? You try the above for weeks or months until you manage to reach customer services to make it work as I mentioned under "as told by NS".

Good luck!

Alternatively, send your card to someone in the Netherlands to have it done for you. This can also be done at many supermarkets and other shops, no need to go to a train station.

See https://www.ov-chipkaart.nl/service-and-contact/service-points-finder.htm?servicetype=3 for all locations.

If sending the card is not an option, NS Customer Service can cancel/block the card altogether. This is permanent, so the card can never be used again.

Just call customer service and let them cancel/block the card as Robert mentioned. It will (almost...) always be cheaper to buy a new card for E7,50 if you need one in the future than to travel back to Holland. It might be a good excuse though ;)