Is there anything I can do to stop someone from posting my article in the internet?

One send a cease and desist letter to the person who is posting your article. Two - contact the hosts of the web sites and insist that the article be removed immediately.

Maybe. Talk to a copyright lawyer about the matter. Can you prove that the article was actually written by you ? Did you publish it somewhere? You may be able to have it taken off the Internet under the Digital Millennium Copyright Act - DMCA. Check out your rights under the DMCA by searching that term and "take down notices". Good Luck!

It depends on where they posted it. You can usually complain to the website that is hosting it and get them to take it down using the DMCA notice provisions that they have (if they have them set up).

If it is really *your* article (e.g. your creative writing), and you have not sold or otherwise granted any rights, you should own the copyright. Before you can enforce it, you must register the work with the US Copyright office. This can be done online. After it is registered you can enforce your copyright. Meanwhile, you may notify the website owners that you own the copyright to the article and that they are in violation. Once you have effectuated notice, they must take it down to avoid liability for the copyright violation.