I believe you cannot sue for damages in the US unless the injured party has actually filed the copyright paperwork before the theft.
If you don't have a valid copyright on file with the government, then I think all you can do is get them to stop using the content. I suppose it's concievable that you might be able to go to civil court over it, but it won't actually be a copyright infringement suit without the paperwork.

disclaimer: I am not an expert (I know patents but not so much copyrights)...please do your own research rather than take my word for it