Hi Jaqi,
First and foremost, we’re sorry to hear that you’re experiencing this.
It is unfortunately a common theme as an affiliate, so you need to police it constantly. I would suggest you start here: Copyscape – Responding to Online Plagiarism
As far as making it more difficult for people to steal articles from you, you can do things like disable right click, etc. but at the end of the day if they really want the content, they can get it. i would advise you to sign up for copyscape and to display their “protected by copyscape” deal on your homepage to deter thieves.
Hope this helps.