document.onkeydown = mykeyhandler;
function mykeyhandler() {

  if(event.shiftKey&&(window.event.keyCode==37) || event.shiftKey&&(window.event.keyCode==39) || event.ctrlKey&&(window.event.keyCode==67) || event.ctrlKey&&(window.event.keyCode==86)){
     /*alert("Función no permitida")*/
     return false;
  }

  if ((window.event && window.event.keyCode == 116) || (window.event && window.event.keyCode == 122)) {
     window.event.cancelBubble = true;
     window.event.keyCode = 8;
     window.event.returnValue = false;
     return false;
  }
}