A few thoughts.. (Score 0)

A few thoughts...

- Work with either local law enforcement (based on the IP) and / or the FBI.

- You may be able to get the ISP to reveal the user's identity be working with the authorities.

- You may be able socially engineer the billing information from the ISP, by acting on behalf of this person.

- With full access to the machine, you may be able to find other revealing information on their network.

- Try to email them a link to a free something, that they won. Make it enticing, but not too over-the-top.