They're called AntiModule in the game files.
That said, I should mention that the antistructure cruisers (at least for TEC) are the only way the AI has a chance of defeating your "wonderfully fortified planets". The AI is absolutely abysmal at taking properly fortified planets; I realize this might be the point, but nerfing them to that level will make the game much easier, even on the highest difficulty settings.
Also, if you're going to make them cost something so extreme as 50 fleet supply, I think the AI would preform better by simple removing them from the player.entity files (PlayerTechRebel.entity etc.). At that extreme fleet supply cost, the AI would probably be better off building any other type of ship, rather than have it waste a huge portion of its fleet supply on ships that can only attack structures. AntiModule frigates are not needed for the AI to run, as after all, the Vasari do not have them.