The good news is "to get lobbed our way" doesn't mean much to a culture that can do distributed projects all over a solar system. A problem along the lines of

The good news is "to get lobbed our way" doesn't mean much to a culture that can do distributed projects all over a solar system. A problem along the lines of whats the ideal cannonball size to genocide the common mosquito? There is also a really nice MAD effect where the window where you can hit a solar system without getting hit back is extremely short, like a century perhaps. Longer than that and we send a probe to the cannonball and turn it into diffuse gray goo that the solar wind blows away when it enters the system, and we return fire 1000 times over, so any logical civilization would send in the asteroid weapons much earlier, like the first time they sniff RF transmissions or the first dark side of the planet electric lights.