It generally isn't illegal for an employee or former employee to criticize a business, but defamation by a disgruntled employee is another story.
Defamation happens when someone makes a statement of fact that is knowingly false and that statement injures the other party's reputation. Oral statements are called slander, while written statements -- like that you'd find online -- are called libel.
Just like a human being, a business can be defamed, and many find themselves targeted where it hurts the most: online, usually on social media or review sites. After all, it only takes a few minutes and a click of the "submit" button to post a derogatory missive that thousands might see within hours or days. Depending upon what was said (and where), false statements can hurt your sales or destroy your reputation in your industry. Even a statement on a site like Glassdoor.com that alleges a toxic corporate culture and other wrongful treatment could cost you long-term contracts and good employees in the future, among other losses.
So, how do you protect yourself online from a bad review? Generally speaking, the sites that host online reviews are under no obligation to check the veracity of what is posted. Nor are they likely to simply take down a review if you just ask. You may also need to act quickly to preserve the electronic evidence that will allow any anonymous poster to be tracked down. For that, you need experienced legal assistance.
If your business is under threat because of a disgruntled employee's outrageous claims, talk to an attorney who understands defamation laws and how they apply to businesses about your options.