You won't stop the bass easily. Sound moves in three ways- directly, through the air (radiation), structure-borne transmission (through materials and objects) and modulation sound through the air causing the structure and objects to resonate strongly enough that they radiate the sound. At -30dB, the second may not be the reason for the complaint, but the last one is probably is. Low frequencies travel a long distance in air without completely sealing the room but the floor and drywall act a bit like a drum head and you can't be sure exactly what part of a floor or wall will be the spot that causes the sound to reach the other people.
Unfortunately for you, the sub may have to sit idle until you find a different place. If you concentrate on having good mid-bass, you won't bother other people in the same building and for a lot of music and other material, it should sound very good but it won't rumble the way you might like during high-intensity action scenes, music with heavy bass or special effects.
The cost to prevent the bass from reaching the other people will be too high to make sense and if you do any construction, you'll need to get permission from the owner.