My question involves a consumer law issue in the State of: California

I have been getting invoices from an accountant that I asked to amend my tax returns. My tax return was complicated this year and he did not know the law well enough for him to amend it, so wanted me to approach a lawyer and get the lawyer to tell him how to amend the report !

I refused, told him that I didn't need to use him if I needed to get a lawyer involved, and that was that. Ever since, he has been sending me invoices for $500 for the tax returns that he never amended !! The latest invoice includes threats to sue me in small claims court and / or report me to the credit bureaus.

Can you give me any advise on how to write him a letter disputing these charges, as he never performed the services he was contracted to do (he never amended any returns !), and also if he can actually report me to the credit bureaus over a disputed bill ? Please advise. Thanks !