Disable "Main Report" on CrystalReportViewer in CR 12

There is a "Main Report" button when the CrystalReportViewer is displayed. What is its purpose? How do I disable it? I've disabled several other items:

viewer.ShowGroupTreeButton = false;

viewer.ShowRefreshButton = false;

viewer.ShowParameterPanelButton = false;

viewer.ToolPanelView = ToolPanelViewType.None;

but how do I turn off the "Main Report" button?