<!DOCTYPE html>
<html>
<head>
<script>
function disable() {
document.getElementById("mySelect").disabled=true;
}
function enable() {
document.getElementById("mySelect").disabled=false;
}
</script>
</head>
<body>
<form>
<select id="mySelect">
<option>Apple</option>
<option>Pear</option>
<option>Banana</option>
<option>Orange</option>
</select>
<br><br>
<input type="button" onclick="disable()" value="Disable list">
<input type="button" onclick="enable()" value="Enable list">
</form>
</body>
</html>