Cannot build correct SELECT in MySql
Try this:

SELECT 
       t.user_id, 
       t.audit_event 
FROM
        (SELECT ua.user_id, ua.audit_event FROM user_audit ua, user_balance_transactions ubt WHERE
        (audit_event = 'RGL_TOURNAMENT_ENTRANCE_PAID' OR audit_event = 'RGL_TOURNAMENT_ENTRANCE_REFUNDED')
        AND ubt.regular_tournament_id='{$tournament_id}'
        AND ubt.audit_event_id=ua.id
        ORDER BY ua.create_timestamp DESC LIMIT 1) t
        WHERE t.audit_event != 'RGL_TOURNAMENT_ENTRANCE_REFUNDED'