How about saving the current selection in a variable and then just comparing that to your condition. If it matches, you can set the ComboBox to whatever you like.
vb.net Code:
Public Class Form1
Private _item As String = Nothing
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
_item = Me.ComboBox1.SelectedItem.ToString()
If _item = "Item I Don't Want" Then
Me.ComboBox1.SelectedItem = "Item I Want"
End If
End Sub
End Class