options for return of garnishment

I have a debt from a hospital that has since been reduced to zero per a financial statement that I had to complete with the hospital. I have several letters from the hospital stating that the account is at a zero balance. (This was from 2008) now all of the sudden my check is being garnished by this law firm. I have contacted them and faxed them copies of the letters. I have also contacted the hospital, which assures me that I do not owe anything. The law firm is refusing to give me my money back. Do I have any options?