How do I remove heroes from the creature banks?

heroes are generated due to the AI mod; this function is enabled/disabled in the map selection menu. As for the adaptive army strength, I don't know; randomness should be at play, so it's either due to the heroes, you're just unlucky, or there's some kind of script (but then these scripts should be enabled on your maps, so I think this is the least likely option).

Ideally, you should conduct a test: attack an object with a weak hero and a weak army, retreat from the battlefield, and attack again with the strongest hero. If the army is the same, it's just randomness; if the army has changed, it's a script.