private void RadRichTextEditor1_SizeChanged(object sender, EventArgs e)
{
    Normalize(radRichTextEditor1.RichTextBoxElement.VerticalScrollBar);
    Normalize(radRichTextEditor1.RichTextBoxElement.HorizontalScrollBar);
}

private static void Normalize(RadScrollBarElement sb)
{
    int maxUser = Math.Max(sb.Minimum, sb.Maximum - sb.LargeChange + 1);

    if (sb.Visibility != ElementVisibility.Visible)
    {
        sb.Value = sb.Minimum;   
        return;
    }

    if (sb.Value > maxUser)
    {
        sb.Value = maxUser;
    }
}