I think the following is a cheese tactic and want to bring it to the devs' attention if it wasn't already. Basically, I've been using my spies on the farms of planets I'm invading the next turn. In essence, in a single turn/"week" I'm able to bring planets with a population of 12 or so billion down to 6 billion. I can easily conquer these worlds with a transport with 3 advanced troop modules or less. Easily! Not sure how my fellow gamers feel about this. I personally love it, though I think it's an exploit and deep inside know it should go away. You guys might want to consider making farms immune to spies.
It just seems highly unlikely that half or more of a world's population can die out in a week/turn due to a single spy wreaking havoc.