Offices are normally rented by businesses, so for the tax appraiser, your situation would be quite out of the ordinary. They appear to be stone deaf to the idea that an office may be rented not for business purposes, and so they lump all office renters together. If they do not want to hear you, all you can do is to keep evidence that:

 1. You are an employee;
 2. You attempted to tell them that you were not liable for the tax because you were not a business

— and present that should they ever attempt to prosecute you for tax evasion or similar.

Specifically, do:

 1. Keep your employment contract, records of them paying your taxes, communication with your bosses telling you what to do;
 2. Send a formal letter (not an appeal) to the tax appraiser explaining the situation

Should the issue ever be escalated, they will not be able to prove that you were a contractor/business (let alone beyond reasonable doubt), and you will be able to recover costs associated with this headache.