Go to ITA Matrix
Click the Advanced Controls link under the destination box
Paste this F ~((DL&BC=E)|(AA&BC=B)|(AS&BC=X)|(UA&BC=N)|(B6&BC=B)|(HA&BC=L)|(HA&BC=K)|(HA&BC=O)) into both the outbound and return extension codes boxes.
Knowing the fare codes is helpful when booking reward tickets, or when trying to predict what the seat price will increase to as the flight fills up (e.g. 2 seats left at this price).
Codes in order from cheapest to most expensive: VXTULKQHSMY