You can try using a small sponge under the strings near the bridge.

Cover the sound hole with cloth or a styrofoam piece.

I don't really know how acoustic guitar could bother neighbours. Do you think they would complained or they have. Anyways, Adriano's advice looks good but I prefer Andy's lol. But This post just crushed my dreams that when i move out it's gonna be ok to crank my amp as loud as I can. snif snif... I must face reality, i'll still hear some TURN THAT DOWN. I hate these words...

Kill your neighbours.

Congtatulations Andy - it was a trick question- you do in fact have have the correct answer. The real trick is to make it look like it was the other neighbours that did it. Then you could sue the builder for his c**p soundproofing;)
Andrew

Plan B is to go electric for a bit with a 300W amp- then go back to acoustic- if there is anything left of your apartment.

lol

I'm with you!

...then cut them up in small pieces and drive deep into the woods and bury them...

I think using a woodchipper on the remains is probably better than burying them... :)

Jim