<select id="mySelect">
<option value="A">Option A</option>
<option value="B" style="display:none;">Option B</option>
</select>
If it's a asp:DropDownList with listitem's you cant't set visible="false" or anything. You will have to rebind the list without option "B".
Or you can achieve the same with javascript (jquery):
function hideB() {
$('#mySelect').find('option[value="B"]').hide();
}