I guess if you know they've done it, then contact them citing your copyright to the material, the date you posted it, and ask them to remove it.
Also, if there is the facility to do so, post a comment to their version of your article repeating this, that they have breached copyright and that the article belongs to you and asking them to remove it. Provide a link to your genuine version of the article.
If this doesn't work, contact the administrator of the site on which your article has been illegally posted explaining the situation and asking them to remove the post.
Hopefully one of these will work.