If the negative press is towards a person rather than a website you could try and build backlinks to some more positive stuff that is already present on the web about them.
For example lets say your client is Bob Smith of Atlanta Georgia, do you find this stuff when you Google Bob Smith Atlanta Georgia? What I would do is build backlinks to the good pages online about Mr Smith, it doesn't matter whose website it is find 10 good websites that are online about Mr Smith, and add links to the pages about Mr Smith.
Also while you're building up some of the other established sites webpages create a few new ones of your own as well, and tag them with negative tags, but have positive comments, use something like is Bob Smith A #@# in the title, and description however, in the text tell a different story.
I see companies doing this all the time when I do background checks on shady sites for clients, and it works. Usually if the site knows what it's doing I will have to search 3, or 4 pages back to find the bad stuff.