If it really really really has to be the Tab key then select those
columns or use a Worksheet_SelectionChange event procedure such as...

Option Explicit
Public rngPreviousCell As Range
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim lLastColumn As Long
lLastColumn = Cells(1, Range("1:1").Columns.Count).End(xlToLeft).Column
On Error Resume Next
If rngPreviousCell.Column < lLastColumn + 1 Then
On Error GoTo 0
If ActiveCell.Column = lLastColumn + 1 Then
Application.EnableEvents = False
ActiveCell.Offset(1, -lLastColumn).Select
End If
Application.EnableEvents = True
End If
Set rngPreviousCell = ActiveCell
End Sub

which keeps the Tabbed-to-cell within the columns from column A up to
the last used column. To get out of that range you will have to select
a cell in the column after the next available column, since the code
relies on the selection of a cell in the next available column to take
the selection to the next row down.

Ken Johnson