Stop adding item in Combo Box

Hi,

I have a combo box with some item. However, when I input an invalid item (that is not existed in the list), it postbacks and add this invalid item to the list.

I want to disable the adding item function but cannot find any properties / behaviour can do it.

Is there any method to stop the adding function? Or, is it possible to check the validation of the inputted value before it autopostbacks?

Thanks.