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Temporary influence immunity wouldn't help them much, and permanent immunity would be potentially game-breaking. I'm thinking that a foreign influence tolerance cap at 1.5x for a gifted planet should suffice. Any more than that and the perceived value should go drastically down, such that you should need to pay them to take the planet off your hands.
I trade one of my planets for all sorts of goodies. I can get technologies, weekly sums of money, trades goods, whatever. It's the same planet over and over. I can get something like 900 credits weekly for this planet. They take ownership, but lose it due to my overwhelming influence. Perhaps the perceived value of a planet should be decreased in proportion with the influence of the originating side subtracted by the influence exerted by the planet itself. It seems a bit odd that the AI d