Mashable had a useful article on How To Prevent and Report Online Stalking. In a nutshell, know that anything you post online could possibly be made public; use security and privacy settings where possible; and know how to report stalking. Don't try to fight back directly, but use proper, authoritative channels.
Also check out the link at the bottom, 3 Tips for Managing Your Online Reputation. Basically, be smart about what you post online but realize that not everyone in your social circle may have the same level of privacy settings on their own profiles. If anything embarrassing does become public, you probably won't be able to get it deleted, but you can continue to practice secure posting so that eventually the good aspects of your reputation will outrank the bad. Finally, be careful about companies that claim to monitor your online reputation.