[Aiken]

Weapon idea: Neurodestructor. This dreadful device obliterates any and all forms of sapient life in the system and leaves colonies depopulated but physicaly untouched. It should be possible to capture 0M pop colony of course, else it would be useless.

Another WMD idea is an asteroid bombardment - so you could establish some monstrous device in the asteroid field and push the field toward a planet.