First, how did he obtain the photos? Did you consent to them? There is no law against posting nude photos of adults in most circumstances in the U.S. And if he's outside the U.S., how would you stop him? Where will he post these photos? We need much more information.
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On the facts you posted, you can't.
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Since the man is threatening you, you have a contact method. You could hire an attorney to write a letter explaining the liability he will create for himself through his actions. Blackmail is a serious offense, etc.
You may want to discuss your situation with a lawyer in more detail. Most lawyers on Avvo, including myself, offer a free phone consultation.
Andrew M. Jaffe
Attorney at Law
Practice Limited to E-Commerce and Internet Law
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.