I also used to get worried when people steal my content, when you run a popular blog, then people will most likely steal your content. But I don't worry about it, just as long as that person links back to my website.
If you use yahoo and bing search you'll realize that ezine articles still rank high, yet people copy that content and put it on their website with a resource box that links to the person who wrote that content.
After seeing this I did some digging and realized that, duplicate content is not really an issue for search engines. Instead the person who steals your content will create a backlink to your website. If you look carefully then you'll realize that the web has a lot of duplicate content that is ranking pretty well on search engines.
So if someone copies your content, that will never affect your keyword rankings. Just make sure you have your on-page and off-page optimization on point to make sure you're the one who ranks well for that keyword and not them.