Cyberstalking and all I have is IP address?

I was wondering if anyone knows what steps I need to take with a cyberstalking case. I am a musician and for the past year, I have been receiving emails from my website from the same anonymous person. The emails are harring, abusive, threatening and at times delusional. The reason it is anonymous is because the person is clicking on the "contact us" link on the site in order to send email to me. They have recently begun to send more email than usual, sounding more delusional and contacting through other links as well. All I have is the person's IP address, which I got from my web server. I contacted the person's ISP and they refuse to help me or even investigate, even though I have sent them copies of the email and let them know this is an ongoing thing. I am planning to fill out a police report in the morning. Is there a way to find out who this is or am I powerless over this if the ISP will not help me?