I do agree with John that just hiding bad coverage is not the sole answer. You should tackle it head on and either respond to it that you messed up or fight your case and have backup for your claims - contribute to the conversation. You must join in!
What I am going to challenge is that it is ok to work on new pieces to push it down AFTER you have dealt with it. Why not try to get good things to show up above it? As long as it is genuine stuff. I am sure that any company that messed up has some good traits and so they should tell the world about it. Press releases and social media are a great way to do so and I have found very quick results with both pushing down blog posts in the search results pages for our company (Sitemasher).
Tip: When doing press releases online, make sure you have a press release distribution service like PRweb.com that has SEO features and use them to the full potential. (ie. Use the title tags for links, link 1 keywords string for every 100 words, tag the release with Technoriti tags etc.)
Also, what is important to know, that when you successfully have good stuff above the bad press (you can maybe get it to page 2 o3 of the search results pages), it is and will always be on the net - so do make sure you respond and comment on it!