A year ago, a nightmare began for Berlin restaurateur Anna Fragos. Her restaurant received excellent reviews from diners on Yelp. But when she refused to take out paid ads, all her positive ratings disappeared. Hairdresser Anika Fehrmann had a similar experience. Her bitter realization: through no fault of her own, she lost her good online reputation and her appointment book started to dry up. And Yelp is offering no help or remedy.