SELECT Distinct ItemNo
FROM [Your Table] as YT
WHERE NOT EXISTS
   (SELECT *
    FROM [Your Table] as T
    WHERE T.ItemNo = YT.IemNo and CostType = "B")

The only problem with the above is that it could be slow with a large table.
Not exists is quite slow.

Two query solution is to build first query that gets all the ITemNo records
that do have a costtype of B and then use the unmatched query wizard to
return those that don't.

** IF** your table and field names don't have spaces or other special
characters you can do this all in one query.

SELECT Distinct ItemNo
FROM YourTable LEFT JOIN
 (SELECT ItemNo
  FROM YourTable
  WHERE CostType = "B") as T
ON YourTable.ItemNo = T.ItemNo
WHERE T.ItemNo is Null

SELECT DISTINCT ItemNo
FROM YourTable
WHERE ItemNo NOT IN
  (SELECT ItemNo
  FROM YourTable
  WHERE CostType = "b")
ORDER BY 1;

Change YourTable  to the actual table name.

If you have a lot of records, this could be slow. Indexing the ItemNo and
CostType fields could speed things up.

I forgot an old rule: If an IN statement is slow, try an EXISTS and visa versa.

SELECT DISTINCT A.ItemNo
FROM YourTable AS A
WHERE A.ItemNo NOT EXISTS
  (SELECT "X"
  FROM YourTable as CT
  WHERE CT.ItemNo = A.ItemNo
  AND CT.CostType = "b")
ORDER BY 1;