trovato il modo

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="JavaScript">
function blocca_f11(){
if(event.keyCode == 122){
{
event.keyCode = 0;
event.returnValue = false;
event.cancelBubble = true;
return false;
}
}
}
</script>
</head>

<body onkeydown="blocca_f11()">

</body>
</html>