Workaround: Set the next style property for all types except the Paragraph to an empty string
foreach (var style in this.radRichTextBox.Document.StyleRepository)
{
    if (style.Type!=Telerik.Windows.Documents.Model.Styles.StyleType.Paragraph && !string.IsNullOrEmpty(style.NextStyleName))
    {
        style.NextStyleName = string.Empty;
    }
}