The easiest (and the evilest) way to defame a guy who is your enemy or competitor is to have an anonymous id and start writing negative about him/her. A similar case happened with Ashish Mehta, a cardiologist in Mumbai who was the victim of defamatory posts on a blog hosted on blogger.com.
The challenges for Google giving this service are many fold.It is practically not possible to discredit or credit an opinion. As a blogger, I may have my reservations on certain issues and may write about it, but it could be 'defamatory' for the guy in the line of fire.
The big question that arises out of this debate is "Can Google be held responsible ?"
I'd say no.
But,certainly yes if the company looking at the case did not take action swiftly and do damage control, which I'm sure Google would have done.
In future looking at the preventive actions, it would be advisable to periodiclly Google your name/business or name/business + review to keep a tap on unwarranted harmful activity popping on search. That could also be called as "Prevent harmful SEO -II". Still controlling content would mean lots of resources. Early detection seems to be the only practical choice in self interest.