you should try to look into the log files.
LOOK AT THE SECURITY LOG FILES – they should give you the ip for the computer that your looking for.
Once you have the IP use a “Whois” utility like http://samspade.org/. You will find the registrar for the domain then be able to contact the user’s ISP (Internet Service Provider, like Verizon Online DSL). The ISP will be able to prosecute or cancel the user’s account.