Hi Sandramilke, and welcome to the NS Community!

If you are moving abroad I would definitely advise you to cancel the subscription.
With € 31,00 a month you probably have an NS Flex contract. This can only be cancelled on the day itself.
To completely cancel NS Flex you'll have to pick up a deactivation at a ticket machine before you leave the country. So my advice would be to cancel on the day you know you won't be travelling anymore, but not so late you won't have time to pick up the deactivation anymore. 

You can cancel your contract completely by contacting Customer Service.

YOu can do it all by yourself by using the mijnNS account that anyone gets automatically-you may have to activate it. It does not work properly on a fone-use a laptop/desktop. In that csae it remains basic=means travel on credit as long as the card is valid, for if you ever return to here and have to use train/bus. But BIG problems will then arise as most likely your bankaccount is also cancelled?

Keep in mind that you will receive a monthly mail with an invoice for 0 euros if you switch to NS Flex basic/without a subscription. 

If you completely cancel the subscription, you shouldn’t get too many mails from NS. Not everybody likes those 0 euro mails.