Yamahaluver,
I would not suggest plugging ports on a subwoofer. The compliance of enclosure may be too low potentially causing damage to the woofer. Most woofers, whether recommended for sealed or ported enclosures, typically require a 25%-50% larger enclosure when ported vs. sealed. If a woofer's recommended sealed enclosure is 1.25 cu.ft., chances are its ported recommendation will be around 2.0 cu.ft or so. With the box being very large after plugging a port, the power needed to reach xmax may be cut in half. Output will be reduced as well, usually by 3db or possibly more, which is what we are after. However, the potential of damaging the woofer due to over excursion or over powering just doubled as well. There are a few risks involed. I would not recommend this solution for most people, as most people do not know limits until they are well surpassed (no offense intended towards any here

).
My assumption in this case is that there was little insulation/sound damping measures taken during the construction phase of your apartment. I would say that this was done as a cost saving measure from the owner's/builder's perspective, thus limiting your ability to enjoy your system. Mtrycrafts is correct, the only way to really fix the problem is lowering the bass output level or finding a new residence.