Public Sub RemoveNumberFormat(ByVal columnName As String, ByVal lastColumn As Integer)
    For i = 2 To lastColumn
        textval = Range(columnName & CStr(i)).Text
        Range(columnName & CStr(i)).NumberFormat = "@"
        Range(columnName & CStr(i)).Value = textval
    Next i
End Sub

RemoveNumberFormat "AM", lastColumn
    RemoveNumberFormat "AU", lastColumn
    RemoveNumberFormat "AV", lastColumn
    RemoveNumberFormat "BA", lastColumn
    ... etc.

Sheets("MailMerge").UsedRange.NumberFormat = "@"