What can be done about internet harassment if I have the individual's IP address?

I recently helped a friend of mine who was diagnosed with cancer set up an online blog to document his battle. He is a pretty well known college athlete so it has gotten a lot of traffic.

However, in the past week, someone has posted numerous harassing comments -- basically death threats, ranging from the simple "I can't wait until you die" to posts that contain much more profane language.

The posts all come from one IP address, so I know the individual's approximate location as well as internet service provider.

What is the best thing to do from here? I don't think there is any real danger, but it's harassment. Is it best to contact the police, or do they have no jurisdiction since it's the internet? Or would it be best to contact the person's internet service provider and let them know that their service is being used inappropriately?
Also, we have already banned the IP address so the person cannot make future postings. We are just wondering how to turn the individual over to the authorities.

You can also do what is called a whois lookup on the IP. It will tell you the ISP. Contact them and complain.

With just an ip address a normal person can trace it back to the corresponding ISP using sites like Ip-details.com .But even then chances are there for that person to use VPN and other IP hiders for such activity.So best is contacting the police autorities.They have permission to greater extent hence they will be able to catch the hackers sooner.