<html>
<body>
<form>
<select id="pets">
<option>Cats</option>
<option>Dogs</option>
</select>
</form>
<p>Click the button below to disable the dropdown list above.</p>
<button onclick="disableElement()">Disable DropDown List</button>
<script>
function disableElement()
{
document.getElementById("pets").disabled=true;
}
</script>
</body>
</html>