I suffered harm from a negative SEO campaign like this.
I was in a relationship with a young woman, whose father is a wealthy CEO of a public company. I will never be able to prove it, but he arranged for my name to be smeared. He ensured that when you google my name, "arrested" showed up in Google instant search. Needless to say, my clients were not thrilled.
He also got the local newspaper to carry a story about an anonymous person in a very specific field of work (mine) who allegedly was being reported for sexual harassment. Again, I can't prove anything, as no names were mentioned, only very direct hints. The newspaper article cost me more than a few friends and who knows how much business.
My lawyer advised me to avoid this guy and recommended that I terminate the relationship with the young woman. I did, and I never told her why.
I have since made several attempts to contact Google to get them to remove the negative search results, but so far I have not received a reply.
I am slowly gathering a pile of evidence of major tax evasion towards her father and his off-shore accounts on Cyprus, and one day revenge will be mine. Justice I have no hope for.