<html>
<body>
<form>
<select id="pets">
<option>Cat</option>
<option>Dog</option>
<option>Horse</option>
</select>
</form>
<p>Click the button below to disable the second item in a dropdown list.</p>
<button onclick="disableElement()">Disable Option</button>
<script>
function disableElement()
{
var sel=document.getElementById("pets")
sel.options[1].disabled=true;
}
</script>
</body>
</html>