function DisableOnContextMenu(ev) {
  return false;
}

function DisableContextMenu(ev) {
  if (ev) {
    if (ev.button && ev.button == 2) {  // W3C DOM2
      return false;
    } else if (!ev.button && ev.which == 3) {  // N4
      return false;
    } else if (navigator.platform.indexOf("Mac")!=-1
      && navigator.appName == "Netscape") {
      return false;
    }
  } else {
    if (event && event.button && event.button == 2) {  // IE
      return false;
    }
  }
}

if (navigator.appName == "Netscape"
  && !(navigator.platform.indexOf("Mac")!=-1)) {
  document.captureEvents(Event.MOUSEDOWN);
}

document.onmousedown=DisableContextMenu;
document.oncontextmenu=DisableOnContextMenu;