Get a lawyer to handle the site--they'll take it more seriously.
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I often say as cocktail chatter that there is no gravity on the Internet, what goes up does not come down.
That having been said, I have worked for dozens of clients to get information removed or search engine de-listed. I would say the success rate for these kinds of actions among Internet attorneys is around 70% and that the cost is from $1,000 to $1,500. It all depends on the circumstances.
If you know who the poster is, another avenue many clients take is to have a lawyer write a cease and desist letter to the person. While a cease and desist letter has little legal effect, it will tell the poster he could get in legal hot water which will cost him a lot of legal fees if nothing else.
You may want to discuss your situation with a lawyer in more detail. Most lawyers on Avvo, including myself, offer a free phone consultation.
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.
Depending on the site, some websites will remove them upon receiving a politely-worded request from you attesting that you are the person who's name is being used, by someone posing as you, who doesn't have authority or permission to do so.
Typically, the websites themselves are going to be protected legally by the Communications Decency Act (47 USC § 230), so threatening them with legal action is unlikely to be effective. You may be able to sue the person responsible for posting as a "john Doe" prior to learning their real identity, and subpoena records about the poster (I.P address, timestamp, email address, etc) from the website owner.