Ho provato a creare un altro Javascript che disabilita tutti e due i tasti:

codice:
<script> 
if (top.frames.length != 0)
{
top.location = self.document.location;
}

var omitformtags=["input", "textarea", "select"]

omitformtags=omitformtags.join("|")

function disableselect(e){
if (omitformtags.indexOf(e.target.tagName.toLowerCase())==-1)
return false
}

function reEnable(){
return true
}

if (typeof document.onselectstart!="undefined")
document.onselectstart=new Function ("return false")
else{
document.onmouseup=disableselect
document.onmouseup=reEnable
}

var mes="Messaggio alert";  

function clickIE()  {if (document.all) {alert(mes);return false;}}
function clickNS(e) {if 
(document.layers||(document.getElementById&&!document.all)) {
if (e.which==1||e.which==3) {alert(mes);return false;}}}
if (document.layers) 
{document.captureEvents(Event.MOUSEDOWN);document.onmousedown=clickNS;}
else{document.onmousedown=clickNS;document.oncontextmenu=clickIE;}

document.oncontextmenu=new Function("return false")

</script>