try
--------------------------------------------
<script language="javascript">
document.onkeydown=function(){
if(event.keyCode==116){
event.keyCode=0;
event.returnValue=false;
}
}
</script>
--------------------------------------------
<script language="javascript">
document.onkeydown=function(){
if(event.keyCode==116){
event.keyCode=0;
event.returnValue=false;
}
}
</script>
if(event.keyCode==116){
event.keyCode=0;
event.returnValue=false;
}
</script>