To add to this, it's not like the AI only has a light frigate shipyard; this is happening at planets with lvl 3 shipyards too.

check the economy tab - the AI will continuously buid things regardless of their economy, so if it can't afford capital ships it will simply spam whatever cheap unit it can afford with it's weekly income. Unfortunatelly there isn't really a way to fix this - you can try abandoning a high-income planet to try to give the AI a chance to rebuld, but you'll still have to deal with the existing stacks, since the AI doesn't sell units.

Would it be possible to hack in a scripted solution? If a script is able to detect when I as the hutts have moved a smuggler fleet over a world, could we have a script that runs periodically, detects a large number of lancers (or whatever), subtracts 30 of them, and replaces them with a reasonable mixture of ships of equal value?