It's only 350 metres wide. That'd be enough to seriously fuck up any large country on Earth without causing much damage, relatively, to other parts of the world. Anyway, you could keep the equipment on the rock, keep Apophis skimming Earth nice and close every few years, and have a permanent very big deterrent.

Apophis is big enough to wipe out a large metropolitan area, but not much more than that, with plenty time to evacuate.

It could, however, excavate quite a crater that could become a very large water reservoir.

But I would prefer to do such major construction projects with nukes - the delivery time and positioning are much more controllable. Also, we have a whole lot of them sitting around and the freedom to use larger formats (they wouldn't have to ride on planes or missiles) could yield much cleaner ones too.

I suppose governments won't like the idea of civilians operating gigaton+ nukes, but that's something to be resolved between politicians and lobbyists.