As far as I know from all my time on the net, and having looked around again after you're query to make certain. You cannot hide your source code.
On another note preventing right-click and highlighting can make legitimate users suffer on your site. Its generally counted as a bit silly. I see your problem however, if they keep taking your posts word for word, there is not much you can do as plagiarism on the net isn't overlly controllable.
Alternatively you could write your text into an image, and post the image up. It would be a bit large, but with no right click they may not be smart enough to get the image.