You have to use Javascript...

Although it doesn't always stop people from taking it!

The following code tells you how to do it and was taken from http://allwebco-templates.com/support/S_add_rightclick.htm

Create a file called something like norightclick.js on your server with the following text inside it:

function noRightClick() {
if (event.button==2) {
alert('You may not right mouse click this page.')
}
}
document.onmousedown=noRightClick

Then in each page you want to stop from right clicking add the following in between the <HEAD></HEAD> tags...

<SCRIPT LANGUAGE="JavaScript" SRC="norightclick.js"></SCRIPT>

No more right clicking (you can customise the above alert ('') to whatever message you want to display (if you so wanted you could even have a random message displayed to add a bit of variety!).