Help the new zookeeper correctly remember the head keeper's strict instructions.

The head keeper was absolutely clear about the rules for the magical creatures enclosure. She strictly warned us that we ______ feed the invisible badgers after sunset.

The correct answer is must not.

When must not (or mustn't) is used in direct speech to express a strict prohibition (Direct speech: "You must not feed them!"), it does not backshift or change in reported speech.

Changing it to "didn't have to" would be a disaster—it means there was no obligation to feed them, entirely missing the fact that it is dangerously forbidden! Furthermore, "hadn't to" is not standard English for past obligations.