Use the disabled attribute in the <sd-optgroup> to disable the section and prevent it from being selected.

```html
<div class="h-[260px] w-[400px]">
  <sd-combobox label="Label">
    <sd-optgroup label="Section 1" disabled>
      <sd-option value="1">Option</sd-option>
      <sd-option value="2">Option</sd-option>
      <sd-option value="3">Option</sd-option>
    </sd-optgroup>
  </sd-combobox>
</div>
```