Devotion Aura unstackable

[NagaRulez]

Good day. I was wondering if how to make devotion aura unstackable? I gave them to all my custom buildings (because its one of their racial traits) and now my units have 22.5 bonus armor.

 

[Jaina Proudless]

If I am to believe this thread you have to put a buff on the spell so it won't stack.