You really can't. There are some Javascript snippets that you could include that will stop the general (uninformed) public, however this will not stop someone from plagiarizing your work.
To understand why, you have to know a little bit about how the web works. When you look at a web page you are not looking at the page on the server. Instead, you are actually looking at a copy of the page on the server that your computer just downloaded.
Every time someone views your website they are copying it! Luckily that doesn't mean they are also plagiarizing it. However once your page has been downloaded, you won't have the kind of control you need to stop it from ever being copied.
And none of the above takes into consideration the fact that someone can simply read off your text and write it down into an editor.
I wouldn't worry about it too much. It's bound to happen that some amount of your work will be plagiarized (at least if it's good). In reality, it probably won't affect you much if at all, and if it does you can always sue for copyright infringement.
There is some Javascript code in the reference if your heart in intent on using it. I would advise against it though, it only hurts the user experience.