<body oncontextmenu="return false"> ... </body>
That being said: DON'T DO IT.
Why? Because it achieves nothing other than annoying users. Also many browsers have a security option to disallow disabling of the right click (context) menu anyway.
Not sure why you'd want to. If it's out of some misplaced belief that you can protect your source code or images that way, think again: you can't.