Cigarette Smoke From Neighbor... What Can I Do?

I own a condo. I am on the end unit, and my one single neighbor is a heavy smoker. Has smoked over 20 years in her unit. We smell it in our unit and it is the worst thing ever.

We have separate heating and cooling systems, and I think I have narrowed it down to how we are smelling it. It is only when our heating/cooling system is not running. The smell comes in through the return vents. We have one downstairs in the livingroom and one upstairs at the top of our stairs.

Any advice on how we can remedy this issue? We tried an air purifier, that didnt help. Anything else we could try?