Taking legal action--going after an individual for posting bad reviews-- is not an option we typically recommend for several reasons.
First, the Streisland Effect--which can bring unwanted attention to you online.
Second, its expensive--very expensive.
Third--while it is easy to sue for defamation or libel, it is extremely difficult to prove it in a court of law (a single emotional outburst by an upset customer does not necessarily fall under "libel" or "defamation").
Lastly even if the feedback is legally considered defamation or libel you still must prove both "where" it was posted (IP address), as well as "who" posted it. The "who" is difficult even if YOU know exactly who posted it--proving it is a different story.
We continue to encourage merchants to calmly respond to negative reviews publicly. Responding honestly and appropriately and then taking a step back from it will demonstrate to online customers that the customer feedback is likely untrue and that you are a professional business capable of handling even the most unprofessional customers.