You can trap window events to handle keystrokes, including stopping Ctrl-C from copying. You'd also have to disable the right-click or they could copy text that way, too.
But (big one)! A user can still click File->Save As and then view the source code that way and you can't prevent that.
Not to mention if the user can see it, they could type it in exactly the same way themself (though tedious, it would work).
I've seen a few websites that now render text as an image - it looks to be happening on the fly, too. That prevents cutting and pasting since there's no text to copy. But it's much slower and would be a lot of network traffic for the image (compared to text). And the user could still type in what they see manually.
If you still want some javascript, let me know.
-Ner