Cops are useless for this unless you're a woman doing the calling.
I just went through the same problem for the past 4 months. Except I'm UPSTAIRS, but the noise downstairs was such that I may as well been living downstairs from extremely noisy neighbors, at all hours.
I basically started retaliating (with noise of my own, and to serious extremes, and acting like a complete psychopath (well after a while it was only half acting!).
They moved out, and even said to my face it was because of me. Whatever, I won, they lost.