Flags is a bit field. Which means you need to query using the bitwise operators. The bit that controls this behaviour is bit 32 (the specific flag is QA_USER_FLAGS_NO_MAILINGS). If it has a 1 then the user does not receive emails. If it has a 0 the user receives emails.
So to find all these users you need to check the users that have that flag (without caring about the rest) set to 1:
SELECT * FROM qa_users WHERE flags & 32 > 0