It usually depends on what the complaint is. If they are flaming you on a forum, you are out of luck, because they own that site and they can do whatever they want on it. If it is something else, (spam, copyright infringement, etc.) you can go to betterwhois.com and see what the whois server is. Go to the whois server's website and contact them by phone or e-mail, and they will usually resolve it for you.
Companies don't want any legal litigation against them, so a lot of times they yank a site for any issues.