As rbobbit and grega stated... you can't lock them out. However, here is a nice JavaScript by Martin Webb. Don't forget they can turn off JavaScript. This script is like leaving your purse in the car and locking the doors. You just keep the << honest >> people out:
// -- Original: Martin Webb (martin@irt.org) -->
// -- This script and many more are available free online at -->
// -- The JavaScript Source!!
-->
// -- Begin
function right(e) {
if (navigator.appName == 'Netscape' &&
(e.which == 3 || e.which == 2))
return false;
else if (navigator.appName == 'Microsoft Internet Explorer' &&
(event.button == 2 || event.button == 3)) {
alert("Sorry, no right clicking allowed here."

;
return false;
}
return true;
}
document.onmousedown=right;
if (document.layers) window.captureEvents(Event.MOUSEDOWN);
window.onmousedown=right;
// -- End