I might be wrong, but aren't those super-stacks supposed to represent a political power that's the same as the Aedui and Arverni? As rebels they can't recruit troops, but the Aedui and Arverni do.Also, without them we would get the Romans/Sweboz/Getai taking over the universe.

However this thread is about the constant-spawn bug, and yes, it does seem to be triggered(in my game) every time the city is besieged when it's controlled by a player faction. I think the best solution to this is to send a single levy unit and fight 10,000 auto-resolves with them and then delete the unit.