They have not reimbursed me as promised

Citi had been charging me an outrageous APR. Upon my asking them to close the account, they offered to bring the APR back down to an acceptable level and refund the excessive interest I had paid for several months. They did bring the APR back down, but they have neglected to refund me the money as promised. I have spoken with customer service twice already, and both times they said they would do it, but my account has not reflected any such credit.