Preventing TextBlock from getting deleted

private void richTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Back || e.Key == Key.Delete)
            {
                TextPointer caretPosition = rich.CaretPosition;
                if (caretPosition != null)
                {
                    TextPointer prevCaretPosition = caretPosition.GetNextInsertionPosition(LogicalDirection.Backward);
                    if (prevCaretPosition != null)
                    {
                        Paragraph paragraph = prevCaretPosition.Paragraph;
                        if (paragraph != null)
                        {
                            Inline tb = paragraph.Inlines.LastInline;
                            if (tb != null) 
                            {
                                Trace.WriteLine(tb.GetType().ToString());
                            }
                        }
                    }
                }
            }
        }}