I support a business who has been targeted by yelp, and it's not pleasant. I can't prove anything, but shortly after turning the abusive sales troll down, we started getting negative reviews. Look up the users making the reviews, and it seems they have a history of making negative reviews. What's more, most reviews were factually and demonstrably inaccurate. We couldn't find any of these users in our system, so we knew they weren't customers.
Now sure, they could have been normal trolls out to do what trolls do, but it just seemed too coincidental that they started popping up after we turned down a business relationship with yelp. Meanwhile, our customers' positive reviews would often never show up on yelp due to their algorithm.
The obvious solution to this entire headache is to dissuade family and friends from using yelp, spread the word far and wide that they are dishonest in their policies and that companies can pay for reviews. As "family IT", we have far more authority than yelp could ever hope for.