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;
		
selectobject=document.getElementById("pureDropDown").getElementsByTagName("option");
  	for(z=0;z<selectobject.length;z++){
		selectobject[z].disabled=true;
		}
 
}