var oSel=document.form1.DropDownCity;
while(oSel.options.length>0){
oSel.removeChild(oSel.options[0]);
}

<SCRIPT LANGUAGE="JavaScript">
function del1(){
var obj = document.getElementById("s");
var count = obj.options.length;
for(var i = 0;i<count;i++){
obj.options.remove(0);//每次删除下标都是0
}
}
</SCRIPT>

<script language="javascript">
function del() {
for (var i = s.options.length - 1; i >= 0; i --)
s.options[i] = null;
}
</script>

<SCRIPT LANGUAGE="JavaScript">
function del1(){
var obj = document.getElementById("s");
obj.options.length=0
}
</SCRIPT>

function removeOption(index)  
{   
    document.getElementById("selUnassignedUsers").options.remove(index);   
}  

function removeOption(index)   
{   
    document.getElementById("selUnassignedUsers").remove(index);   
}