Ok, so, I thought that the AI warped in reinforcement ships via the warp gate. But is there another way?
I have a pretty worthless AI planet between two of my own. On the way to capturing the further one of mine, I neutered the worthless one, and I destroyed everything but the command center and two of the Wormhole Guard Posts. I even destroyed the warp gate. This planet is a bit of a throughfare for my ships, so I want to keep it free of AI ships so they don't harass my own, but everytime I destroy them, they keep coming back. Also, I control both planets that ships could possibly be traveling from, so it's not reinforcing from another planet. Where are these ships coming from?