Invoice of ns flex which is not used anymore

Hello,

I was an exchange student in the Netherlands from October to December 1st. I had a subscription when I was there for Ns Flex trains.Before 1st of december I have sent a mail which says that I don’t need that montly subscription anymore due to moving from the Netherlands. However, several times I have received me invoices after 1st of december.I didn’t even use that card for trains as I have already left the Netherlands.But still NS tell me or send me emails that I have to pay invoices several times.Before leaving the Netherlands,I have stopped my subscription by email and NS Page.I won’t pay anything for no use that yellow card.Could you please help me and stop my subscription again?