JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.2/css/bootstrap-select.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.2/js/bootstrap-select.min.js"></script>
<select name="dropdownBranch" id="dropdownBranch" class="selectpicker" data-live-search="true">
  <option value="0">Choose Number</option>
	<option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
</select>
<select id="pureDropDown">
  <option value="0">Choose Number</option>
	<option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
</select>
<button onclick="disableDropdown()">disable</button>

JavaScript

function disableDropdown(){
      	var selectobject;
      	selectobject = document.getElementById("dropdownBranch").getElementsByTagName("option");
      	selectobject[3].disabled = true;
        $('#dropdownBranch').selectpicker('render');
    }