private void RadRichTextBox_CommandExecuting(object sender, Telerik.Windows.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is PasteCommand)
    {
        e.Cancel = true;
        this.radRichTextBox.CommandExecuting -= this.RadRichTextBox_CommandExecuting;

        RadDocument documentFromClipboard = ClipboardEx.GetDocument().ToDocument();
        RadDocumentEditor editor = new RadDocumentEditor(documentFromClipboard);
        DocumentPosition pos = new DocumentPosition(documentFromClipboard);

        foreach (var block in documentFromClipboard.Sections.First.Blocks)
        {
            documentFromClipboard.Selection.Clear();

            pos.MoveToDocumentElementStart(block);
            documentFromClipboard.Selection.AddSelectionStart(pos);

            pos.MoveToDocumentElementEnd(block);
            documentFromClipboard.Selection.AddSelectionEnd(pos);

            if (!documentFromClipboard.Selection.IsEmpty)
            {
                editor.Copy();
                this.radRichTextBox.Paste();
            }
        }

        this.radRichTextBox.CommandExecuting += this.RadRichTextBox_CommandExecuting;
    }
}