Good luck trying to get it removed. I’m going through the exact same problem. I was marked hired by someone who didn’t hire us and he gave us a 1 star review. Never worked for him ever. Thumbtacks great advice was to reach out to the customer and ask him to update the review. Review of what??? I said something derogatory about Thumbtack and they took down my post. So my advice to you is don’t waste your time asking Thumbtack for help and just right a reply to the review so your future customers see your side of it.
Honestly, a competitor could do this and ruin your reputation. Thumbtack needs to work for us Pros, too, not just be rigid about their policies. We pay them to work for us and bring us possible business opportunities. Saying they'll NEVER fix an obvious error seems unreasonable business practice. Most businesses will look at each situation individually.