Dim BlockedRange As Range

Private Sub Worksheet_Change(ByVal Target As Range)
    If WatchDog = False Or BlockedRange Is Nothing Then Exit Sub
    If Not Intersect(BlockedRange, Target) Is Nothing Then
        Application.EnableEvents = False
        Application.Undo
        Application.EnableEvents = True
        MsgBox "Íåëüçÿ ìåíÿòü ïðîøëûå ïåðèîäû!", vbExclamation, "Âíèìàíèå"
    End If
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim cell As Range, Blockeddate As Date
Set BlockedRange = Nothing
Blockeddate = WorksheetFunction.EDate(Date, -1) + 1
If Not Intersect(Target, Range("a:f"), UsedRange) Is Nothing Then
    For Each cell In Intersect(UsedRange, Columns(3))
        If IsDate(cell) Then
            If cell < Blockeddate Then
                If BlockedRange Is Nothing Then
                    Set BlockedRange = cell.Offset(, -2).Resize(, 6)
                Else
                    Set BlockedRange = Union(BlockedRange, cell.Offset(, -2).Resize(, 6))
                End If
            End If
        End If
    Next
End If
End Sub