Even as an active attorney, I cannot give you specific legal advice unless you are my client. But I can express my opinion and give general legal suggestions. If you have a specific legal question, please consult with a competent legal practitioner in your area of the world. In my last post, I talked about splogging. If you find that your work has been copied by someone without your permission, you may wish to send them a notice of their copyright violation. Here is a suggested form:
Copyright Violation Notice: Please be aware that the publication of the __(insert description)___ is a violation of the copyright owned by ___(insert the name of the owner)____ and violates Title 17 of the United States Code (if you live in another country, you can insert the law that governs copyright in your country). Your violation of the copyright may result in legal action and could result in a claim for monetary damages. Continued publication of this copyrighted material will result in additional claims and damages. You are requested to remove this ____(insert what should be removed) immediately.
There are lots of different wording that might be used. I do not think it is productive to threaten legal action, unless you are actually intending to file a lawsuit. However, you can certainly notify people that their actions could result in legal action.
Here is a link to a very good discussion about what to do in the case of a copyright violation.
Do not just suffer. Be proactive.