Exclude certain line depending on a certain condition in t-SQL - sql-server

Question

Firstly, I have created the following stock sales SELECT statement:
SELECT
PostST.TxDate AS TxDate,
PostST.Reference AS InvNum,
Client.Name AS CustomerName,
SalesRep.Code AS SalesRep,
WhseMst.Code AS Whse,
CONCAT (StkItem.Description_1, ' - ',StkItem.Code) AS Item,
_etblLotTracking.cLotDescription AS LotNumber,
CASE
WHEN PostST.TrCodeID = 34 THEN (PostST.Quantity * 1)
WHEN PostST.TrCodeID = 30 THEN (PostST.Quantity * -1)
ELSE 0
END AS QtySold,
CAST
(
CASE
WHEN PostST.TrCodeID = 34 THEN (PostST.Credit / PostST.Quantity)
WHEN PostST.TrCodeID = 30 THEN (PostST.Debit / (PostST.Quantity * -1))
ELSE 0
END
as [money]
)
AS CustomerPrice,
StkItem.ItemGroup AS ItemGroup,
StkItem.ulIIStockType AS StockType,
YEAR (PostST.TxDate) AS Year,
DATENAME (MONTH, (PostST.TxDate)) AS Month
FROM
PostST
INNER JOIN StkItem
ON StkItem.StockLink = PostST.AccountLink
INNER JOIN PostAR
ON PostAR.cAuditNumber = PostST.cAuditNumber
INNER JOIN Client
ON Client.DCLink = PostAR.AccountLink
INNER JOIN SalesRep
ON SalesRep.idSalesRep = PostAR.RepID
INNER JOIN WhseMst
ON WhseMst.WhseLink = PostST.WarehouseID
FULL JOIN _etblLotTracking
ON _etblLotTracking.idLotTracking = PostST.iLotID
WHERE
PostST.TrCodeID IN (34, 30) AND StkItem.ItemGroup <> 'TRAN'
This query is run from Sage Evolution and data is dumped into Excel. From there I manipulate to view each item and the sales history in quantity per month and year.
My problem is as follows:
Within the data that follows, is sometimes a customer who has perhaps stopped buying a particular item. What I need is for this statement to also filter out a line by the following condition:
If the customer does not have a sale for 6 months or more, then that STOCK ITEM must be filtered out for that specific customer.
Thank you!