#1. You could have a helper workbook that opens the real workbook. If macros
are disabled, then this helper workbook won't be able to open the real
workbook. (But even then the user could disable events.) I don't have a
workaround for that.
#2. It copies and pastes the formatting from the the hidden worksheet to the
real worksheet. If the user pastes a general 1.1 into a cell, when they move to
another cell, the old format on that hidden sheet is pasted into real worksheet
(and sets it to the way it was).
Another option if you can get by that macro stuff.
You could catch the change, save the new formula/value, do an undo and then
reapply the formula/value.
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim myFormulas As Variant
On Error GoTo errHandler:
myFormulas = Target.Formula
With Application
.EnableEvents = False
.Undo
End With
Target.Formula = myFormulas
errHandler:
Application.EnableEvents = True
End Sub
But that breaks if macros are disabled or events are disabled.