[Spoo]

That gives me an interesting idea. After a large portion of the popualtion is killed off (say 50%) the planet is automatically infected with a level one plague caused by the massive amounts of dead bodys laying around, but the plague will go away by itself after a year. I'm not sure of the strength of a level one plague, but it shouldn't kill off what remains on the planet. However, it would make immediate colonization of the planet quite difficult for several turns, as the colonists would all die every turn.

For another idea, if a planet is "glassed" then half of it spaces for facilities should be dissabled to reprissent perminant damage done to the planet.