If customers leave an unfair review diffuse the situation by responding to their comment (pointed by red arrow in screenshot below)—
Why does this matter? Because reviews live on forever. If your site has 100s of products with 1000s of reviews I suggest starting with your top selling items and working backwards.
This might seem like a labor intensive project but if you break it down based on most important products and create a system around it you will quickly be able to work through the list.