So, I got the random event where class 25 planets appear. One showed right next to one of my existing planets, well within its sphere of influence. However, in a monumental act of poor etiquette, my close friends the Terrans sent a colony ship they had buzzing around to settle it.

Now, I don't want to go to war over this, but even if I reload the game I can't beat them to it. My understanding is that culture-flipping is highly impractical, and they probably won't sell such a planet cheaply. But if I could kill our open borders agreement, that might stop them (although maybe not).