[Ombudsman]

Dispute the charge with your credit card immediately. Clearly Delta is not unlikely to budge on this unless you keep going up the "food chain" to higher levels of management and by then it may be too late to initiate a chargeback. This way you only have to make your case once with the CC company and let them do battle with Delta's evil minions. Save your confirmation e-mail and receipt as well as it will show what fare the ticket was booked into so you can show it was refundable should you ultimately need to take this to court.

Airline pricing is tricky. If this was a multi-segment itinerary and the agent that booked it made an error an booked just one segment with a non-refundable fare it will render the entire ticket as non-refundable. If this happened it will work in your favor as well. What person in their right mind would actually request 3 of four segments with a refundable fare and 1 non-refundable.

Good luck and hope all works out in your favor.