This isn't really Yelp's fault. Sites like this always have fake reviews mingled in with those that are real. The solution?

Assume people with a 4-5 star review are competent. GET MULTIPLE QUOTES. Doesn't matter if they have 5*, if you feel something is off with them they you avoid.

You also will find out when a quote is too cheap/too expensive and reduce your risk to getting conned.

Don't blame the internets or Yelp. Blame youself for not doing due diligence.