Unfortunately air filters that truly filter smoke are not cheap.
My BlueAir 402 with smoke stop filter does a wonderful job, but at $120/6 months, it ain't cheap.
You mentioned that your landlord claimed that your ventilation is isolated from your chain smoking neighbor. Keep complaining to your landlord. And submit a written complaint, even better, submit it with your rent check.
Otherwise, sniff around for other places of leakage. Honestly, most floors and walls are not sealed. I'm pretty sure the smoke is also seeping in through the walls, through the studs, joists, and where the wall meets the floor. If your floor is carpeted, you can pull it up at the edges, and look for a gap between the wall and floor. If possible, you can get some minimally expanding foam in a can, and squirt some stuff to fill the gaps, then push the carpet back into place. I had to do that in my place.
Another tactic, (rarely successful, as most smokers I've come across in my area are rude f#$4rs) is to ask politely that they smoke outside, go for walk away from the premises, and enjoy the outdoors while they smoke (I had success with one neighbor with this one, but alas he moved). If they refuse (which they probably will), you can find something that will annoy the crap out of your neighbor to perhaps force a kind of "compromise". (Bag pipes for a little Scottish warfare, perhaps?)
Although I'm not sure what you could do that won't get you in trouble with the law. Perhaps if your plumbing is linked, you can flush the toilet every time your neighbor showers? Just throwing out some ideas.
I feel your frustration. Some people just don't give a damn about others health. Why should they? They don't give two craps about their own health.
OK, now the Pro-Smokers can flame away...
-Ed