Can I file a file a lawsuit against an orthodontist who refuses to reimburse for services not rendered?

I've had breaces for approximately a year. I've been keeping regularly scheduled appointments and making timely payments. I have made an overpayment on my account that would have covered 4 months. I called to have my braces removed. When I called demanding a refund of an overpayment to my account. I was informed that I would only receive a small portion of the overpayment amount. Therefore, I have been charged for a month's visit despite asking to have my braces removed. What should I do?