Add Delete Button To Macro

Hi guys the following formula works great and if text is written over then it will put the original text back in automatically.

only problem is if i hit delete on the keyboard it deletes it - i want to stop this delete button from working.

any1 know how? please see macro belo

Private Sub Worksheet_Change(ByVal Target As Range)
Dim sel, sv
Dim rng As Range, cell As Range, plan As Range
Dim rng2 As Range, cell2 As Range, WO As Range
Set sel = Selection
sv = sel.Value
Application.EnableEvents = False
On Error GoTo e
Application.Undo
Set rng = Range([A1], [IV1].End(xlToLeft))
For Each cell In rng
If cell = "Plan" Then
If plan Is Nothing Then
Set plan = Intersect(cell.EntireColumn, ActiveSheet.UsedRange)
Else
Set plan = Union(plan, Intersect(cell.EntireColumn, ActiveSheet.UsedRange))
End If
End If
Next
If Not plan Is Nothing Then
Set rng2 = Application.Intersect(Target, plan)
If Not rng2 Is Nothing Then
For Each cell2 In rng2
If cell2 = "WO" Then
MsgBox "You cannot change " & cell2.Address(False, False)
GoTo e
End If
Next
End If
End If
sel.Value = sv
e:
Application.EnableEvents = True
End Sub