As is explained the forum thread I mentioned, all you can do is to clear the whole text by pressing [ctrl] + [a] or selecting the whole text and then pressing [del] key if there is not an item whose text exactly maches the text after deletion,

For example, suppose there is a WebDropDown which has the following items:

<ig:DropDownItem Selected="False" Text="가나" Value="1"></ig:DropDownItem>
<ig:DropDownItem Selected="False" Text="가나다" Value="2"></ig:DropDownItem>

And suppose you've just typed "가나다" in the drop down's text box.

You can delete "다" by pressing [BackSpace] because there is an item whose text is "가나". Then the text becomes "가나".

But you cannot delete "나" successively because there is not an item whose text is "가". All you can do is to clear the whole text "가나".

This is exactly the WebDropDown's behavior when its EnableCustomValue is false.

If this behavior doesn't satisfy your requirements, you should use it with EnableCustomValue true.

Thanks for the reply, but it did not help.
We need delete without 'ctrl + a'.