If you have a complaint about your tax affairs then you can either take your case to an independent conciliation service attached to each main tax office, or to the independent Ombudsman at the Ministry of Finance.
Be aware, however, that even though you may contest an assessment, you are still required to pay your taxes, unless you are otherwise given a specific dispensation by your tax office.
If you have a complaint then clearly, you should try and resolve it with the tax officials in the first instance.
If the matter cannot be resolved to your satisfaction, then you should ask your tax office for the contact details of the conciliator fiscal.
You can also obtain contact details by visiting Conciliation Service.
If the matter remains unresolved then you can contact the national Ombudsman, the Médiateur du Minéfi.
The web site can be found by visiting Médiateur du Minéfi.
The only other recourse is to a court of law.
Next: Index - Personal Taxation in France