You could also do this if you prefer a less imperative approach.
C#
(from object item in listBox2.Items
where !listBox1.Items.Contains(item)
select item).ToList().ForEach(x=>listBox2.Items.Remove(x));
VB
(From item In listBox2.Items Where Not listBox1.Items.Contains(item)item).ToList().ForEach(Function(x) listBox2.Items.Remove(x))