In the military logistics module which is synced to the AI Commander module, in there you can set the force pool. I've never actually tried to set it to 0 but I'm fairly sure that's an option.
The way factions reinforce is through ground/air units, depending on how you have it set up (whether that be to "side" or "side and faction") and also dependent on how the faction is made (for instance, when set to "side," if you're using Unsung or ISIS that don't have any helicopters, they'll only get reinforced by the assets they have available, so in this instance it would be by land. If set to "side and faction," they'll use air assets from NATO or CSAT, and their vehicles, etc).
The reinforcements from what I can tell just sort of spawn in the mission. Maybe off so somewhere? Basically, the military commander will request battle casualty replacements from their force pool if they hold military objectives, and the size and delay of which can be set as you've already seen (i.e. Seldom which I think is when they're at 50% of their original force size, or Packets, which I think is when they're at 75% of their original force size, etc etc). If you set forcepool to 0 (again, pretty sure that's an option) they shouldn't replenish their units at all.
Take a look at this for probably a much better explanation:
http://alivemod.com/wiki/index.php/Military_Logistics