How to disable ctrl key combinations using Javascript?

function disableCtrlKeyCombination(e)
{
//list all CTRL + key combinations you want to disable
var forbiddenkeys = new Array('a','n','c','x','v','j');
var key;
var isCtrl;

if(window.event)
{
key = window.event.keyCode; //IE
if(window.event.ctrlKey)
isCtrl = true;
else
isCtrl = false;
}
else
{
key = e.which; //firefox
if(e.ctrlKey)
isCtrl = true;
else
isCtrl = false;
}

//if ctrl is pressed check if other key is in forbidenKeys array
if(isCtrl)
{
for(i=0; i < forbiddenkeys.length; i++)
{
//case-insensitive comparation
if(forbiddenkeys[i].toLowerCase() == String.fromCharCode(key).toLowerCase())
{
alert('Key combination CTRL'+ String.fromCharCode(key) + ' has been disabled.');
return false;
}
}
}
return true;
}

//=====================
< body onkeydown="return disableCtrlKeyCombination(event);"> < /body>