INT gives Mana as we all know.
By gaining INT (for Heroes), it will surely gain Mana as side bonus (be it the Hero has INT as primary attribute or not).
Meanwhile, items can also grant Mana by granting the Hero unit an additional INT or a direct +Mana ability.
World Editor > Advanced > Gameplay Constants... > Hero Attributes - Mana Point Bonus per Intelligence Point: 0.00
Well now, you will have a 0 bonus of Mana Point per INT.
Now it's about item giving a direct boost of Mana Point.
Well, just create another item ability that does not give you either INT or Mana Point since that is your objective, right ?