For those that encounter the same problem with hackers (we did today), you can register to Google analytics (
http://www.google.com/analytics), install the Javascript they generate in your website pages and find out in Goggle Analytics reports from which ISP/geographical area/network the hacker comes (plus, you got excellent analytics for your website analysis...) and report to that ISP abuse email about this hacker.
If you don't want to touch your website code, you can also simply use one of the WHOIS or better the GeoIP free services (e.g.
http://www.whos.com that has both WHOIS and GeoIP lookups,
http://www.networksolutions.com/whois for WHOIS,
http://www.maxmind.com for GeoIP etc.) to find the same thing quickly: run GeoIP lookup for the IP and you will get the same info that Google Analytics provides and then do WHOIS to find the abuse enmail of the ISP and report.
Good luck!