A Dark Void ban would only affect Darkrai (and Smeargle, but Spore is so much better on him), so if you wanted to nerf Darkrai without a complex ban, that'd be the way to do it. Even then, though, you've still got a Pokemon with incredible speed, sky-high SpA, near perfect coverage between its STAB and the unreliable but powerful Focus Blast, access to Nasty Plot, and passable defenses to boot. Even without the sleep move, that's still a tall order. On the other hand, having to rely on Hypnosis to abuse sleep and Bad Dreams would make it a bit easier to handle.