public void DisableUnwantedExportFormat(ReportViewer ReportViewerID,string strFormatName)
{
    FieldInfo info;
 
    foreach (RenderingExtension extension in ReportViewerID.LocalReport.ListRenderingExtensions())
     {
        if (extension.Name == strFormatName)
        {
             info = extension.GetType().GetField("m_isVisible",BindingFlags.Instance | BindingFlags.NonPublic);
            info.SetValue(extension,false);
        }
    }
}

并在page_load上：

DisableUnwantedExportFormat(ReportViewer1,"PDF");