Southern Dental ruined my credit

I had two wisdom teeth removed last summer. I was charged 500.00 and paid cash before I walked out of the door. Two months later I received a notice from a collection agency that I still owed 500.00. I went back to Southern Dental, and they agreed that I owed nothing. They promised to correct the matter. I still get notices from the same collection agency. My credit is wrecked, and I can't get the financial aid I need to stay in college. Southern Dental refuses to return my phone calls.