You must pursue the domain name dispute resolution policy through one of several approved providers. The official list includes the National Arbitration Forum (in the US) and the World Intellectual Property Organization (WIPO).
You can also pursue a trademark case through the courts. This approach can be very slow. In the United States, the Anti-Cybersquatting Consumer Protection Act of 1999 makes it easier to pursue a case against someone who registers the name of a famous person.
The quickest solution is often a simple letter from a trademark attorney letting the domain name holder know that you intend to pursue the matter. In the case of most "cybersquatters" who already know they are in the wrong, this is enough to get the domain registration transferred to you.
For more information, check out the legal FAQ of the excellent Bitlaw site, as well as the website of ICANN (Internet Corporation for Assigned Names and Numbers). All domain name registrars belong to ICANN and agree to abide by its Uniform Domain Name Dispute Resolution Policy.
I am not a lawyer and cannot give legal advice. If you have serious legal concerns, consult a lawyer.
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