From: http://wiki.starsautohost.org/wiki/Guts_of_bombing
Quote:

Normal bombs add linearly; that is, adding another bomb always increases the amount of stuff killed/destroyed the same amount. (That is, until the enough bombs are there such that the planet is wiped clean, after which point additional bombs add nothing).

In the event that the number of (standard) bombs required to kill all the people is lower than the number of bombs required to flatten all factories, but the number of bombers would allow all infrastructure to be eliminated.

Do the bombs stop after wiping all population - or after everything is dead/flat?