I think this is the shortest version:
Here's longer version which blocks other things also: Disabling right-clicking by using JavaScript | My Cyber Universe
JavaScript:
<script>
window.onload = function() {
document.addEventListener("contextmenu", function(e){
e.preventDefault();
}, false);
}
</script>
Here's longer version which blocks other things also: Disabling right-clicking by using JavaScript | My Cyber Universe