Option Explicit
 Dim Old_value
 Dim New_value

 Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Old_value = Target.Cells(1, 1).Value
 End Sub
'==============================================
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
On Error GoTo Final_Step
If Intersect(Target, Range("A1:F12")) Is Nothing Then GoTo Final_Step
   New_value = Target.Value
    If Old_value = "" And Target.Cells.Count > 1 Then
     Application.Undo
     GoTo Final_Step
    End If
     If Old_value = "" Then
      Target.Value = New_value
      Else
     Application.Undo
    End If
Final_Step:
 Application.EnableEvents = True
End Sub