Prevent user from deleting a particular sheet

As far as I can tell, it isn't possible to natively tag a single sheet as non-deletable; and there isn't an event that can be used to detect when a sheet is about to be deleted so the workbook can be protected preventively.

However, here is one potential workaround:

1. Protect workbook structure: this will, as you indicate, prevent all sheets from being deleted.
2. Create a "Controls" sheet. On this sheet, maintain a list of all sheet names (except those you don't want to be deletable).
3. If users want to delete a sheet, they will have to select its name on the Controls sheet (e.g. in a data validation drop-down menu) and press a "Delete" button. This button will call a macro that temporarily unprotects the workbook, deletes the selected sheet, and then reprotects the workbook.

Of course, the users will have to get used to this way of deleting sheets (as opposed to just right-click > Delete on the sheet's tab). Still, this isn't crazy complicated.

As for how to achieve #2 i.e. maintaining that list of sheet names, I suppose you could make use of a UDF like this one (must be called as an array formula):

Function DeletableSheetNames() As String()
    Application.Volatile
    Dim i As Long
    Dim sn() As String
    With ThisWorkbook
        ReDim sn(1 To .Sheets.Count)
        For i = 1 To .Sheets.Count
            With .Sheets(i)
                If .Name = "DataEntry1" Or .Name = "DataEntry2" Then
                    'Don't include it in the list.
                Else
                    sn(i) = .Name
                End If
            End With
        Next i
    End With
    DeletableSheetNames = sn
End Function

You cannot stop users to delete a particular sheet but you could use the Workbook_BeforeSave() event to prevent the workbook from being saved if a particular sheet is missing. The documentation on this event precisely shows how to allow saving a workbook only when certain conditions are met. See http://msdn.microsoft.com/en-us/library/office/ff840057(v=office.14).aspx

I can prevent a sheet from being deleted via the Worksheet_BeforeDelete Event as follows:

Private Sub Worksheet_BeforeDelete()

    Call ThisWorkbook.Protect("password")

    Call MsgBox("This sheet cannot be deleted.", vbExclamation)

End Sub

This protects all sheets from being deleted, however if you add some event code on the ThisWorkbook module like the following :

Private Sub Workbook_SheetActivate(ByVal Sh As Object)

    Call ThisWorkbook.Unprotect("password")

End Sub

I will then be able to delete any other sheet as soon as it is selected.

Bear in mind, you will lose copy and paste functionality between pages due to the page unlocking when it is selected.