If you already have the outline/subtotals applied, you can protect the worksheet
in code (auto_open/workbook_open??).

Option Explicit
Sub auto_open()
   With Worksheets("sheet1")
       .Protect Password:="hi", userinterfaceonly:=True
       '.EnableOutlining = True
       .EnableAutoFilter = True
   End With
End Sub

It needs to be reset each time you open the workbook.  (excel doesn't remember
it after closing the workbook.)

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm