There is nothing you can do about stopping people from copying your content. There are 100s of silly ameatur techniques like disabling right click with javascript, but there is no foolproof method of doing so.

People may suggest displaying the content with the help of flash, java applet etc. but I am sure you want the search engines to be able to read your content as well.

Google do penalise sites for duplicate penalty, but they are most likely to identify the original and duplicate. I don't think it is a duplicate comtent penalty, since you dont copy any articles.

My suggestion, look for other possiblities of drop in rank. Also you can embed some links to your own site/ other articles in your site in future articles you write.