<form name="frmG" action="#">
<select name="selG">
<option value="0">--------</option>
<option value="1">アイテム1</option>
<option value="2">アイテム2</option>
<option value="3">アイテム3</option>
</select>
<input type="button" value="無効化" onclick="fSelectDisabled(true)" />
<input type="button" value="有効化" onclick="fSelectDisabled(false)" />
</form>
<script type="text/javascript">
function fSelectDisabled(bool){
document.forms["frmG"].elements["selG"].disabled=bool;
}
</script>