You want to cripple an AI with your spies. NO Problem. Take your spies an put them on all AI farms.Wait for one turn and then move to another. The AI will probably send his spies against one of your planets, thus he will no nullify all yours (or will only nullify part of them). Since the ai usually do not build farms for nothing you get the picture: 1 spy == 3 Million+ colonist per turn. No better way to scrap and AI economy.