So what do you do? You write positive blog posts about your client or you hire other bloggers to do the same. You write articles optimized for your client's main keywords and submit them to highly-trafficked sites in your client's industry. You promote them via social media. You make sure the new posts are Dugg, Stumbled, Sphunn etc. You feed a good news press release about your client into the social media vortex and let it take hold.
Once the ugly page has been pushed off the front of the SERPs, you continue to watch the blogosphere closely for any negative comments about your client and make sure they don't rise to the surface again.