@KyuubiNoKitsune-Hime: Did a marvelous turn explaining how we, with our current technology, could redirect a comet's trajectory. I'll just add that any interstellar species presumably has (at least) fusion power as the basis of their drives and energy production. Assuming no new physics, no use of exotic matter, just the incredible amount of energy from fusing hydrogen, you could do more than move a comet. You could use a robotic craft that was little more than big deuterium tank and a fusion rocket, land it on Ceres, then steer it right into the target of your choice. We'd see it coming, but so what? We don't have the technology to stop it, even if we nuked it to pieces. A rain of pieces would get the job done just as well.
If you want to go stealthy though, comets are the way to go. Comets move pretty quickly, and they're not as massive as a dwarf planet or giant asteroid. It's hard to imagine that a species contemplating interstellar hostilities will not have the ability to land a small robotic craft on a comet, and steer it. The craft then refuels from the comet itself, then hops off to find another suitable kinetic weapon.