If I understand correctly, this is what you want:
A drop down has values 1, 2, 3, 4, 5
First time, you click on down arrow of the drop-down, it shows all values in the list.
You select 4.
Now again you click on down arrow of the drop-down, it shows 4 as the selected value.
What happens now: shows value 1, 2, 3, 4, 5 in list
What you want to do:show values 1, 2, 3, 5 in list. Get rid of 4 from the list, just because it's selected.
My solution:
What you want to do is not the standard behaviour of drop down control. Not to mention, it will just confuse the users.
You can not do it, because if you remove it from the list, it can not be selected.
In ASP drop down, as far as I know, it can not be done. I found a work-around in html drop down. See if that helps you. As I change the format from drop-down to list and list to drop-down in these two button clicks, may be you can do that on click. Sorry, can not post better example as I'm running out of time.
<select id="T">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="mercedes">Mercedes</option>
<option value="audi">Audi</option>
</select>
<input type="button" önclick="T.multiple = 'multiple';" value="b1" />
<input type="button" önclick="T.multiple = '';" value="b2" />