Wouldn't you need to solve the n-body problem to hit a planet (earth) from many light years away with a rock? And to do this without exact knowledge of planetary bodies in our star system. And even if you could, a stray hydrogen atom hitting your rock early on would make it miss its target by some light hours. So you better account for every speck of dust on the way and everything with gravitational pull on your rock. An all the quantum fluctuations..
I would even wonder whether the space itself is fine grained enought to precisely target stuff light years away.
If humanity was doing this, we'd likely attach propulsion and accelerate the entire way anyways. Unless there's some new drive technology discovered by then or we hijack something natural/environs to do it for us.
Not many resources really. They can simply use their abundance of energy to hurl, propel, or divert some rock at us at some non-negligible fraction of light speed, and we would be powerless to stop it. Likely we wouldn't even see it coming. Make it two or a dozen to be sure.
It doesn't even take a very advanced civilization to pull off such a thing. Give it maybe a hundred years and humanity might be able to do it too.
Since it is a rather easy to end a civilization in such a way and it is near impossible to defend against, you arrive at a very... interesting game-theoretic problem: Did they already send kinetic weapons our way? Should we destroy them before they decide to destroy us? If we're about to die anyways, does it even matter if we fire back?
It's like the cold war except nobody is able to talk to another and you don't see the nukes coming.