Try this - the macro filters to exclude accounts beginning BS and deletes everything, which leaves only accounts beginning BS
Sub Delete_all_except_BS()
'set variables
Dim lastRow As Long
'find last row and last columns
Sheets("sheet1").Select
Range("A1").Select
Selection.End(xlDown).Select
lastRow = ActiveCell.Row
'filter to exclude all accounts starting BS
ActiveSheet.Range("A1:F" & lastRow).AutoFilter Field:=2, Criteria1:="<>BS*", _
Operator:=xlAnd
'delete all the visible rows and this will leave only those beginning BS
Selection.Offset(1).SpecialCells(xlCellTypeVisible).Delete Shift:=xlUp
End Sub