Refund did not recevie after 3 months and many calls and promises from NS!!Very unsatisfied customer

The train subscription is cancelled per Dec. 2014, but at Jan. 5, 2015 NS still booked Eur318 from my bank account, thus, I instructed my bank to transfer the Eur318 back to my bank account. I thought the case is closed. However, I received a letter from NS about payment default, and demanded to pay the Eur318 first, then there will be an automatic refund of Eur318 to my account in 30 days. I did what was NS requested, and transferred Eur318 to NS at Jan. 9, 2015. However, the refund was never occurr at Feb. 2015. I have been calling and explaining at least 4 occassions, and was again promised about the refund will come. Up to now, nothing happened!! The last 2 conversations were with Mr. Terdstra at Mar. 14, 2015, and with Van Der Molen at today (Mar. 28, 2015). I have prove from my bank slips about the payment transaction.
NOW I AM OUT OF PATIENT!! PLEASE CONTACT ME ASAP, OTHERWISE, I WILL BRING NS TO THE COURT!!!