There is no need to go to a ticket machine, NS is able to initiate to block the card, your card becomes useless after blocking though, as you cannot go to the ticket machine. There is no need to in your situation. Contact Customer Service. There is no other way. Chatting is free. Maybe you are in luck with a  small refund, but that is not a promise.

I guess you can block your card on the ovchip card website of the Netherlands and then it will cancel the Subscription 

Thanks everyone for answering. In the mean time, after more than one hour I finally reached someone from the customer services through chat. I was told that nornally I need to go to the NS machine for finalizing the cancellation. But since I'm not in the country, they can cancel my cancellation without requiring me to go to the machine. I was told also that I do not need to send back the OV chipkaart. It all seems a rather simple procedure. I just wish that they had had followed this simple procedure two years ago, without me paying the subscription fee for two more years and worring the consequences of late payment… 

The system in the Netherlands is that you renew automatically. It does not matter whether you use your subscription or not. It may seem weird to Americans, but this is the way the cookie crumbles.

But the card is blocked and that is the most important.

There is a severe upcoming storm in the Netherlands and all train traffic will be canceled early in the afternoon, so that is why it took an hour to reach Customer Service.

Blocking your card (without renewal) via OV-chipkaart does NOT cancel any subscription, unless you inform NS that the card is blocked/cancelled.