In [Admin > Field Management], a field cannot be hidden. The field is showing that it is being used by certain assets (e.g. emails), however, you have checked and confirmed that the assets do not have references to the field.
If you are certain that the field you are trying to hide is not being used by any assets, yet it is shown as being used by other assets in Marketo, please raise a support ticket.
Marketo Support can verify if the field still has any references within an asset. If not, Marketo Support can delete the dependency in the database so that the field can be hidden.