Try something like this (I used a couple of buttons to toggle between enabled/disabled - you will probably want this in the OnSelectionChanged event)
private void Disable_Click(object sender, RoutedEventArgs e)
{
Job.Items.Add("NA");
Job.SelectedValue = "NA";
Job.IsEnabled = false;
}
private void Enable_Click(object sender, RoutedEventArgs e)
{
Job.Items.Remove("NA");
Job.IsEnabled = true;
}