Beginning Thursday, warning signs appeared when users tried to access the pages of some businesses with five-star ratings, with a button to click saying "Show me the reviews," the Los Angeles Times reported.
"We caught someone red-handed trying to buy reviews for this business," a red-bordered alert box says. "We weren't fooled, but wanted you to know because buying reviews not only hurts consumers, but also honest businesses who play by the rules. Check out the evidence here."
Yelp said it has flagged nine businesses for paying people to write fake, positive reviews, gathering evidence of what it calls "rogue solicitations," including Craigslist postings offering $10 to $200 for a positive review.
"This pretty much breaks every rule in the book, not to mention it's just wrong to mislead consumers with fake reviews," Yelp said on its blog. "To combat this, we've put on our detective hats, tracked down these rogue solicitations and are now giving you a heads up."
Yelp said roughly 1 in 5 reviews are prevented from appearing on its review pages because its proprietary algorithms tag them as possible fakes.
The alerts linked to the nine businesses will be removed after three months, Yelp said, unless there's evidence of any of them still trying to buy reviews.