Online harrassment is often a difficult problem to stop. In the first place, it's hard to define and not all states recognize it as a crime. Also, people who use the Internet as a method to stalk or harass someone may be savvy enough to hide their identity. Statistics on cyberstalking are hard to come by, but women are frequent targets. It's also a tool used by race-based hate groups. E-mail is one vehicle used by stalkers, but they can also use instant messages and chat rooms.
Researchers at Florida State University have developed a way to make monitoring cyberstalking cases easier. Professor Sudhir Aggarwal says the Predator and Prey Alert (PAPA) system allows police to watch and securely record victim's online interactions.

Other links:

Stalking resource center
Wired Safety's cyberstalking report form