Hi Rita,
Unfortunately, this is not currently possible.
AtoM is purposefully designed to be very flexible and permissive, to allow for infinite customization and different uses depending on local conventions. I have seen users create collections that include fonds-level descriptions below them; I've seen users use repository records as collection-level records; Australians use the series as their top-level; and we have to account for users creating new custom level such as sub-sub-sub-fonds. Because we can't predict how users might want to implement AtoM based on their local conventions or how they might customize the levels of description, we don't organize the levels of description in any hierarchical manner that would restrict users from adding lower levels however they choose.
Instead, I recommend developing internal documentation on arrangement and description best practices, and making sure your staff review these before working in AtoM.
It might be possible to develop something like this, where you first organize the terms hierarchically in the taxonomy, and then that is enforced during records creation. However, this would require significant analysis and development to implement, to ensure it would remain performant over time, and to make sure users can disable the feature as needed.
Regards,