Use a disabled spellbook.
Basically add the aura to a spell book (the spell book is an item ability, you must make it a unit ability and give it to the unit in question with the aura added to it). Then using triggers, disable the spellbook ability (not remove, disable). The spellbook icon will be gone from the units command card but the aura will still be active.
This works for every passive ability.
Incase you still do not know what disabling an ability means, it is a techtree related concept whereby instead of physically removing an ability you just turn it off. This lets you turn it back on later when you want it to be available. Other objects like units and technologies can be disabled like abilities and all three of these concepts were demonstrated in the campaign.
The bug works due to disabled spellbooks only disabling the spellbook command card and not the abilities within it.