Do a "whois" domain lookup at:
Allwhois.com - whois domain name search & lookup
This should give you the registrant's name, contact information and address along with the website host.
At any rate, even if the information seems rather anonymous or if it's privacy protected, you should still find a contact email address or telephone number for the hosting provider whereby you can contact them. They'll take care of the rest.
Before you contact the hosting provider, be prepared to provide some background information and documented proof to substantiate your claims. Copies of any communications, emails sent or received, etc.
It will be hard to pass up on the immediate gratification you'd no doubt glean from administering a sound thrashing with a rubber hose, but keep on taking the high road on this. Be professional in your communications and reasonable in your requests and you'll make some progress.
Stick to the high road.
Good Luck.