Roger,
You need to get the IP address of the server. This can be done a variety of ways, for example, open a dos prompt on Windows and do "ping
http://www.site.com" and it will say something to the effect of pinging 10.10.1.184 or something like that. Copy that sequence of numbers then go to
www.arin.net. On the right of that page there is an input box to "Search Whois". This is different than a regular domain whois as it is a whois for IP addresses. Paste the IP address into that box then click search. Hopefully it isn't on a major network and it will just take you to a page that shows who is responsible for the IP block, usually an ISP or hosting company and it will show an Abuse contact. Shoot them an email with all the info you have and the log files and hope they do their job. If the IP is part of a big network it will say to search some other sites whois registry, go there, enter the IP again in their search box and then you should get the correct information for the abuse contacts. Then do as before.
Good luck. Also, don't expect anything besides an automated reply. They, including the company I work for, almost never respond as it is generally unnecessary once the problem has been rectified or violated their privacy policies.