<form action="#">
<select id="selG2">
<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="fSelectDisabledDOM(true)" />
<input type="button" value="有効化" onclick="fSelectDisabledDOM(false)" />
</form>
<script type="text/javascript">
function fSelectDisabledDOM(bool){
document.getElementById("selG2").disabled=bool;
}
</script>