Arteeem,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<select>
<option value="test">test</option>
<optgroup label="Города" >
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</optgroup>
</select>
<script>
document.querySelector('[label="Города"]').disabled = true
</script>
</body>
</html>