How to remove value from ComboBox if its is saved after selecting in Windows Forms Application

hi, I have a 2 combo boxes in which first contains subjects like geography,history,english,hindi.and second on contains type of sections like primary or secondary. so now problem is that if i am appointing a teacher then if i am selecting history subject from first combobbox and primary from second combobox den when afterwards the subject history should not be seen as it is assigned to one teacher..