So, can we not do that Ornn passive change?

First, if people want to be idiots, then just let them.

Second, I like to build the upgraded Abyssal (as Ornn) second item if I'm against a lot of AP, since it gives so much MR. It lets me get a good balanced build without spending 3k gold on Spirit Visage to get more MR early in the game. I can finish up my armor items after upgrading Abyssal. And you're just taking that away.

At least let Ornn build the items himself whenever he wants. Let your team build them later.