If all they did was buy the domain name, there is not much you can do. Anybody can register any domain name without fear of copyright infringement. Your copyright does not extend to holding the domain name with your copyrighted name. The only part that is illegal is cyber squatting, registering domain name and attempting to sell them at an inflated price. It would also be illegal to hold the domain name hostage if it is not used. You can also buy the name if it is not been used for one year.
If the owner of the domain is using the name in a way to violate your copyright, you can sue in federal court. This would include posting a website to disrupt your copyright or attempting to direct potential business from you. You may wish to talk to a local attorney and see if your copyright has been violated.
