Re spying on farms, it reduces pop to the planet's max capacity without that farm on the next turn. If the planet was below max, it might have no effect. I hate the exploit, personally, and never use it. I imagine the spy is nullified as soon as a unit is availble to do the job.
I just loaded up an old save and tried it out.
Yes, the very next turn, the population drops to the new maximum cap. It's the same as if you decommissioned a farm on one of your own planets.
Many of the spies I placed (for only one turn) did not get nullified. So either the AI didn't have the spies to spend, or else it didn't think it necessary. In any case, by the time they have the chance to nullify it, the people are already dead.
I took an 18 billion AI planet down to 6 billion in one turn using spies. That's pretty impressive. That's worth doing even if you don't plan to invade the next turn. That's a huge economic loss.
If you think about it though, the population birth rate is pretty ridiculous. The speed at which time passes in the game isn't the same for everything. So there is no reason to think that the death rate isn't similarly balanced by the game designers. It's impossible that the designers did not consider what happens when a farm is disabled. I don't categorize this as an exploit because it seems to be working as designed.
Edit: On second thought, I've never had the AI disable farms on my own planets before. So given that the AI doesn't take advantage of this functionality, it could be considered unfair to use yourself. Since the AI does have a huge economic advantage, it could absolutely devastate your economy if it so desired. Every spy you can't afford to nullify is billions more dead the following turn when it gets moved to another farm.