private void MainDemoControl_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
{
    if (e.KeyboardDevice.IsKeyToggled(Key.Insert))
    {
        this.isInsertKeyPressed = true;
    }
    else
    {
        this.isInsertKeyPressed = false;    
    }
}
private void radRichTextBox_CommandExecuting(object sender, CommandExecutingEventArgs e)
{
    if (e.Command is InsertTextCommand && this.isInsertKeyPressed)
    {
        this.radRichTextBox.Delete(false);
    }
}