WPF listbox items

Question

Hi all,

I have two listboxes. One gets loaded with student info when I select a subject in the combo box. From the 1st listbox I select a student and pass it to the 2nd listbox. My problem is that for instance I select a subject and the students appear in the listbox and I assign them to the 2nd one. Now if I select the same subject from the combobox it gives me the students again even though I had already assigned them. Is there a way I can delete the records of the students once they have appeared in the listbox once. I am using 3tier architecture and my students are loaded from the database so I use ItemsSource to populate listbox and combobox. I hope this is clear.

Thanks in advance