They won't give me back my refund because it was for lower class

I'm so disappointed, today I phoned this airline (kulula.com) and I spoke to lady named SINAZO who was very useless and rude I told her I want to cancel my flight and I will need my refund ... and she told me they can't refund me because the ticket I booked it for lower class, it was a cheapest...I'm not sure what's that supposed to mean but I found it very ************** and it my money... I really demand my refund back and I won't stop until I have it, help!