The same thing they do now, let it potentially damage the restaurant's reputation. I've seen literally the most fucked up reviews drag down an otherwise 5-star restaurant's ratings...shit like: "The chef's tasting menu was a great price, but it was far too much food...how much sushi do they think two people can eat?" and "My waitress just had the ugliest sweater when I went in today, completely ruined my usually delicious burger! 1 star."

I really wish they had a more quantitatively factored upvote/downvote/reputation system.