Dashboard agenda board pick filter

Hi! We use a personal agenda for each of our Monday users, that filters items out of boards that they are tagged in (e.g. meetings, deadlines, etc). They also use these agenda’s to add items in these boards. Doing this, they need to choose the right board to add an item in. Now, for each of these boards they can choose for adding an item in their agenda, they can also choose subitems. However, I would like these subitems not to be shown as an option. Could there be a way to filter these out and not have them as an option?

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Thanks for your help!