Simply write "(c) Your Name.  Do not copy."  Putting your name on it leaves them a way *in* in case they want to contact you (you can put your email address there also) for terms of re-use.  The "do not copy" (or similar desired "term of use") puts your explicit intentions on the document, in this case reserving all rights to yourself.  I personally put the latter part in a different, legal-looking font so that it stands out as a "meta-item" of the document.

Otherwise, if you think the "cat's out of the bag" already an someone has your work, the best way to protect it is to publish it on the web, where the community can now help you with fair credit assignment.