I have tried BNn, Fczbkk's suggestions and neither solved the problem. Gelatin's mentioned script seems to work but impacts the main window too-you can't see the list of people who clicked on "like" on a post. I have also tried Yahoo Answers idea to repeatedly click on x's. They disappear momentarily but then again appear next day.
What it works is to install the Firefox add on FB Purity. It cleans up the sponsored page, the People you may know and has settings on what you want to appear on Facebook page. I will update my answer if it stops working.