Here are two very helpful guides that will show you step by step how to make Facebook more secure.
http://www.allfacebook.com/2009/12/facebook-privacy-new/ - This is a great article on the new (i.e., less private) settings in Facebook and how to increase the security levels.
http://www.facebook.com/help/?safety - This is Facebook's online security center. You don't have to login to use it. This is a good page to scan now and then so you know what's in it if you need it.
In security, an ounce of prevention is worth a pound of cure. Choose your friends carefully, limit the personal information you publish and keep your password strong and safe. These simple proactive measures will go a long way to enhance the safety and effectiveness of Facebook.
Read more: http://www.site-reference.com/articles/General/Stopping-Unwanted-Communications-in-Facebook.html#ixzz0lagNbg1y