​x
 
<html>
<head>
<script type="text/javascript">
function disableOption()
  {
  document.getElementById("pear").disabled=true
  }
</script>
</head>
<body>
​
<form>
<select>
  <option id="apple">Apple</option>
  <option id="pear">Pear</option>
  <option id="banana">Banana</option>
  <option id="orange">Orange</option>
</select>
<input type="button" onclick="disableOption()" value="Disable pear option">
</form>
​
</body>
</html>
​