1. Say angry things about you in their forums; and 2. Give your business a bunch of negative (and totally irrelevant) reviews. Even if they haven't visited it. We believe it is called "astroturfing"? Or just "trolling"?
In the past 24 hours, Parker's Table 6 has gotten six one-star reviews that have nothing to do with the restaurant in question, but instead just personally attack the chef, calling him things like "arrogant," "d*bag," and "jackass." He was also accused of having "horrible facial hair" and looking "like a video game wizard." Burn?
Yelp has, in the past, promoted its "review filter" that's designed to catch "fake, shill or malicious reviews," but bizarrely it isn't particularly effective at catching a mini-flood of one-star reviews. These negative reviews will probably eventually be flagged by the community and then deleted, but it doesn't reflect well on the legitimacy of any of the reviews found on Yelp. With no checks in place to make sure that a Yelper has in fact frequented an establishment, it's inevitable that you'll have shills, the irrational, or people with axes to grind.