A Google AdWords Help thread has a short question and short answer on how to deal with slanderous or negative Google ads about an individual or company.
The question was, "How do you remove a false statement posted about an individual?" AdWordsPro Sarah explained that in these cases, you need to fill out a Google AdWords feedback form. Once you submit the form, Google will have someone manually review the ad and take action, if necessary.
Sarah explained, Google's "policy team will manually review the ad and pull it down if it is not meeting any of our guidelines."
Now, negative or slanderous search results that are not ads are harder to deal with.
Forum discussion at Google AdWords Help.