You can't freeze them, although you could most likely build code to set the worksheet index to 1, everytime they are changed. The other option would be to hide the tabs via code, using xlveryhidden.
Add this to the Thisworkbook level of the vbe to prevent a change to a different worksheet.
Private Sub Workbook_SheetDeactivate(ByVal Sh As Object)
ActiveWorkbook.Worksheets(1).Select
End Sub
HTH
Cal
PS- I just did a test, and it stops them from changing to ws other then index 1, but index 1 can be changed by draggin the sheet tabs. I would suggest using a sheetname instead. Then protect the workbook to prevent changing of the name.
Private Sub Workbook_SheetDeactivate(ByVal Sh As Object)
ActiveWorkbook.Worksheets("Sheet1").Select
End Sub