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…
If you live in the United States, once you have enough information, you can use the IRS's whistleblower program to report him. You can get up to 30% of the penalty and tax money that the IRS collects if you wait until the idiotic budget sequestration is lifted.
Just make sure that you have solid information, because: "The IRS is looking for solid information, not an “educated guess” or unsupported speculation. We are also looking for a significant Federal tax issue - this is not a program for resolving personal problems or disputes about a business relationship."