Beat them into silence with a flamethrower. But really? You can go upstairs and knock and venture something along the lines of: "can you turn down your TV volume, I'm trying to sleep".

Or, if ya wanna be subtle (haha), you can have really, REALLY loud sex. Or, uh, pretend to. }:> Then leave them a note asking if they like loud noise at three in the morning.

But that won't work if they're perverts who like listening in.