Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<select size="6">
  <option value="BMW">BMW</option>
  <option value="Mercedes">Mercedes</option>
  <optgroup id="myOptgroup" label="Swedish Cars">
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
    <option value="koenigsegg">Koenigsegg</option>
  </optgroup>
</select>

<p>Click the button to disable some options in the dropdown list.</p>

<button onclick="myFunction()">Try it</button>

<script>
function myFunction() {
    document.getElementById("myOptgroup").disabled = true;
}
</script>

</body>
</html>