Technically, every work that you submit is automatically copyrighted (assuming of course that it is a legitimate work). That said, there's no way that individuals or corporations can prevent being plagiarized. You just have more options as a corporation typically due to their retention of attorneys.

As a sole content producer, you can choose to press charges against those who plagiarize against you; however, it may not be worth the legal costs. Aside from confronting the plagiarists and setting the record straight, I'd save the legal action for a plagiarist that has deep pockets, if you know what I mean :)