<!DOCTYPE html>
<html>
<body>
<form>
<select id="mySelect">
<option>Cats</option>
<option>Dogs</option>
</select>
</form>
<p>Click the button to disable the dropdown list.</p>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
document.getElementById("mySelect").disabled = true;
}
</script>
</body>
</html>