var ErrMsg = "Sorry! no right click please";

function disableRightClick(btnClick)
{
  if (navigator.appName == "Netscape" && btnClick.which == 3) // check for netscape and right click
   { 
     alert(ErrMsg);
     return false;
   }
  else if (navigator.appName =="Microsoft Internet Explorer" && event.button == 2) // for IE and Right Click
   {
     alert(ErrMsg);
     return false;
   }
}
document.onmousedown = disableRightClick;