Combo box question

How do you reset a combo box back to an unselected state? For example if I want to disable a combo box based on a user's selection how do I remove the value so that there is no value selected?

I know I can use this code to select a value:

oComboBox.Select(val,SAPbouiCOM.BoSearchKey.psk_ByValue);

Is there a similiar way to unselect something?