Unless you registered a domain name with a trademark in "bad faith," there is nothing they can do about it. They can take you to court, but the boon of online legal issues is that it's such a black and white legal domain, you can represent yourself and make them spend all the money.
If you get an email, ignore it. If you get physical mail, respond that you will take them to court if they do not stop trying to domineer your domain name because you registered it in good faith. You then want to tell them that the nature of your website does not violate any trademark law because your website is a <insert fansite/unofficiality/parody/unrelated to the trademark/etc etc etc>. The more legal terminology you use, the more you're letting them know that you are aware that your website will stand up in court and they should back down.
If they take you to court, PM me. I will tell you what to say and what you don't have to answer. You can represent yourself.
Remember to counter-sue them for foregone income and PR related damages.