The way to avoid this mega-event is to not stockpile more than 4-5 spies at a time.
Can we get some confirmation or correction here from folks who've had the spy-sprawl numerous times? I buy the theory that game state influences timing and choice of mega events, but I routinely keep 9-12 spies in the bank and I don't see this event very often.
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Well since I've never had more than 2 spies at once (ever), and I've had this event a few times, its definitely not 100% correct.
But I do believe that playstyle has some influence on triggering events. I would lean more toward a view that the spy event is triggered when you are holding a HUGE advantage over the AI, and are perhaps prolonging a victory. But thats just my thoughts. Every time I get the spy, its usually if I'm stockpiling ships and growing a booming population/econ for a higher final score. Then the game says, "No more, sir. Just finish the game"