Our oldest child is 15 and likes to visit chat rooms. She does this at the local library because our kids cannot log in by themselves at home (our PC is password-protected). It seems she started chatting with a 28-year-old man. About a month later, she received a phone call from him. Our daughter has refused to give us any information on this man. How do I protect her?
Step one is succeeding at some form of authentic communication with your daughter. If she is withholding information from you about the man on the Internet, she could just as easily withhold information about a face-to-face encounter with someone, or about any of a hundred things. Your challenge here is not the bogeyman on the Internet -- it's the failed communication with your daughter. That's where I'd put my attention, maybe even seeking help outside the family (e.g., family counselor).
I also propose to you that by this age, there is no information about men and the world that your daughter needs to be protected from. Information is now her armor. I'd suggest you and your daughter might benefit from reading How to Get Through to Teenage Girls, from my book Protecting the Gift.