If you want to end NS Flex completely, i.e. travel on credit again, then yes, this needs to be confirmed at a ticket machine.

If you switch to NS Flex Basis, then the change does not need to be confirmed at a ticket machine. This will effectively end your subscription, but you can/will still travel on invoice on full fare. And even if you don’t travel anymore, NS will still send you an invoice for 0.00 euros every month.

(I see you created a topic yesterday about changing your subscription. For the record: changing from one NS Flex subscription to another NS Flex subscription does not need to be confirmed at a ticket machine either.)