Disable ListBox item in WPF

Hi Guys, I have a simple ListBox laoded in WPF & I want to disable some items. How can i do this?

<ListBox Margin="10,55,0,0" x:Name="listBox1"  FontSize="18" SelectionChanged="listBox1_SelectionChanged"
            	HorizontalAlignment="Left" VerticalAlignment="Top" Width="130" Height="200"  Background="GhostWhite"/>

ListBox values:

           public TreeviewTest()
        {
            InitializeComponent();
 List<String> listOfNames = new List<String>() { "Animals", "Plants", "Nature", "Science", "People" };
            Binding myBinding = new Binding();
            //set binding parameters if necessary
            myBinding.Source = listOfNames;
            listBox1.SetBinding(ItemsControl.ItemsSourceProperty, myBinding);
}

I want to disable 'Nature' & 'People' in the ListBox. Thank you.

Karthik