1.SelectionMode 改成可以多选
2.利用KeyDown事件：
        private void listBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Delete)
            {
                for (int i = listBox1.SelectedItems.Count - 1; i > -1; i--)
                {
                    listBox1.Items.Remove(listBox1.SelectedItems[i]);
                }
            }
        }