Settlement/Advance payment overcharging

3 years ago I paid extra on my account with the intention of prepaying my installment. The agent advised this is not possible and they cannot refund the money so the extra funds reflected on my statement every month as an advance amount. They said it will be deducted when vehicle is paid off. I eventually decided to settle the vehicle and it turns out they gave a super poor offer and took my advance amount. This has been a horrific experience. and to date no response from emails sent.