There's also "rival company" reviews. I've heard from some business friends in the Hotel business that rival Hotels are probably leaving bad reviews on Yelp. And there's pretty much nothing you can do about it.
You look at a review, you back-check the date and try to line it up with customer records. You look for complaints, you ask staff if they remember the customer. Etc. etc. Ultimately, its a phantom, clearly a made-up review designed to hurt your company for some reason.
That's more of a thing on Yelp where if you bad-mouth a neighbor with a fake-review, your own business across the street benefits. Less of a thing on Amazon (where a fake negative review won't necessarily lead to customers buying your thing).
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Back 20 or 30 years ago, a fake negative review on a newspaper or an advertisement would lead to suing the perpetrator for slander or libel. Today, there's no recourse at all. And more-and-more people are learning about the marvels of the modern internet age where you can pretty much slander your opponents and benefit from customers as a result.