To the best of my knowledge, it’s nearly impossible to prevent someone from copying/stealing the content from your site. It is possible, though, to find the culprits.
Copyscape offers a service that identifies other sites that are plagiarizing your content. Just enter the web page url from your site that you’d like to check and click “go.”
Copyscape will return any pages that match significant parts or all of the content for the requested page.
The free version allows you to conduct 50 “searches” per month.