How To Disable Some Items Of A Combobox

private void Form1_Load(object sender, EventArgs e)
{
    UnselectItem();
}

public void UnselectItem()
{
    this.comboBox1.ValueMember = "Value";
    this.comboBox1.DisplayMember = "Text";
    this.comboBox1.Items.AddRange(new[] {
    new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=0},
    new ComboBoxItem() { Selectable = true, Text="Selectable1", Value=1},
    new ComboBoxItem() { Selectable = true, Text="Selectable2", Value=2},
    new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=3},
    new ComboBoxItem() { Selectable = true, Text="Selectable3", Value=4},
    new ComboBoxItem() { Selectable = true, Text="Selectable4", Value=5},
     });
    this.comboBox1.SelectedIndexChanged += (cbSender, cbe) =>
    {
        var cb = cbSender as ComboBox;

        if (cb.SelectedItem != null && cb.SelectedItem is ComboBoxItem && ((ComboBoxItem)cb.SelectedItem).Selectable == false)
        {
            // deselect item
            cb.SelectedIndex = -1;
            MessageBox.Show("You cannot select this Item");
        }
    };
}
//User Defined class for Combobox Item with the Selectable property

private class ComboBoxItem
{
    public int Value { get; set; }
    public string Text { get; set; }
    public bool Selectable { get; set; }
}