Hola, prueba así:
Saludos
Código (php) [Seleccionar]
DELETE FROM attempts A INNER JOIN users U ON A.id_user = U.id_user; WHERE A.id_user IN (SELECT A.id_user FROM attempts A INNER JOIN users U ON A.id_user = U.id_user GROUP BY A.id_user HAVING COUNT(*) >= 6) AND U.login_banned = 0
Saludos