Case "nb", "b"
             ' ...
         Case Else
             MsgBox "Неверное значение"
         End Select

Private Sub Worksheet_Change(ByVal Target As Range)
      If Target.Count > 1 Then Exit Sub
      If Intersect(Target, Range("B5:B30")) Is Nothing Then Exit Sub

      Dim sComm As String

      With Target
          Select Case .Value
          Case "nb", "b"
              sComm = InputBox("Ввод причины:", "Ввод причины", "Прогул")
              If Len(sComm) > 0 Then
                  .ClearComments
                  .AddComment sComm
              End If
          Case Empty
              .ClearComments
              Exit Sub
          Case Else
              .ClearComments
              .ClearContents
              MsgBox "Информация", 0 Or 64, "Заголовок"
          End Select
      End With
End Sub