I'm pretty sure the AI spends just as much on reinforcements for a planet whether you neuter it or not; guard posts then apply a bonus to the amount spent so it gets converted into more ships. So, the best thing to do is to alert neutered mark I and II planets, to prompt the AI to spending it's reinforcement budget as inefficiently as possible. At least, I know that neutering planets has no effect on the spillover of reinforcements -> waves.
