objSelect = document.getElementById("selProva")
objColl = objSelect.options
do
objColl[0] = null
while (objColl[0] != null);



dove la select è questa

<select id="selProva">
<option value=""></option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>