A positive approach.
Send a private message to the person who posted negative review and ask them to remove it because the review doesn't make sense. If they still refuse to remove the review, move to the next step.
Going the legal way.
First, reply to the review about how fraud the review is, post screenshot of email conversation.
Then, block the user. (With your personal account)
Now, report the negative rating to Facebook as a "harassment / personal attack / spam"
Wait for Facebook to do their job. They will review your post and will remove the rating.