It is impossible to display text as text, allow it to be copies, but stop it from being changed once it is copied. Once the text is on the clipboard, you have lost control over it. I do have a couple ideas for you to think through though.
The first thing I would do would be to look at why you are trying to protect your text. Is it really necessary? The whole point of a website is that you are sharing information. What people choose to do with that information, to an extent, is up to them. Even if this were technically possible, they could still transcribe the data to Word and then manipulate it.
If you really feel that certain text must be protected and must remain unchanged, you could convert that text to an image. Then they could download the image but they couldn't easily manipulate the text. Here is a tip on how to do that:
Convert Text to Image[
^]
This isn't a solution you want to do for your whole page, but it could work for sections that are going to remain static anyway.
You could also create a user agreement that has to be agreed to before the visitor can see your page. Something simple that says "I will not modify the text I see" or something to that effect.