Javascript Example
Screen shot
Enable or Disable Dropdown

<html>
<head>
<title>Drop Down Disable</title>
<script language="javascript" type="text/javascript">
function enable_disable_dropdown()
{
if(document.getElementById("click").checked==true)
{
document.getElementById("country").disabled=true;
}
else
{
document.getElementById("country").disabled=false;
}
}
</script>
</head>
<body>
<h3>Click on Check for Enable or Disable Drop Down</h3>
<input id="click" type="checkbox" onClick="enable_disable_dropdown();"/>Click Me
<select id="country">
<option selected="selected" value="">-- Select Country --</option>
<option value="1">India</option>
<option value="2">Germany </option>
<option value="3">France </option>
<option value="4">Poland</option>
</select>
</body>
</html>

You can download source code here