Unfortunately, any number of suggestions that we can offer up won't be any more effective than the most simple one -- getting your neighbor to turn the bass down. Too bad they've decided to be pricks about the noise, because there are ways of working around these kinds of bass issues.

The thing to keep in mind is that bass frequencies behave differently than the midrange and highs. Their wave lengths can be longer than the room itself, which creates a lot of interesting variations in how they sound from one location to another. Most prominently, the bass waves can easily pass through walls, and room-induced wave interactions can result in bass cancellations at one location and extreme boosts at another location. Because of this it's entirely plausible for the bass to sound louder in your room than in your neighbor's room.

Aside from adjusting the subwoofer level down, changing the location of the subwoofer can make a difference, and filtering it so that the bass dropoff occurs at a higher frequency are two approaches that the neighbor can take to limiting the bass that intrudes into your space. But, these are far from surefire approaches, and changing the location of the sub can actually make things worse if the conditions are right.

Unfortunately, short of going to masonry construction, there's no easy way to keep the bass from passing through the walls if they adjoin one another. Going to the association board is probably the easier solution.