A 66-year-old man with chronic obstructive pulmonary disease (COPD) presents for review. He has a 40-pack-year smoking history and remains breathless on exertion. Spirometry shows FEV1 55% predicted with FEV1/FVC ratio 0.55. He is still smoking 10 cigarettes per day and uses salbutamol via metered dose inhaler as needed. What is the single most important intervention to reduce long-term mortality and disease progression?