When a character activates it, all active abilities should be stopped. If there is a reason this doesn't happen, I'd love to know, because it makes Kennan unstoppable.

It makes it so you cannot move or cast any skills, and it interrupts channeling. Kennen's ultimate isn't a channel. It's also why Karthus' E works during Zhonya, and I think there's a couple others...Fiddlestick's ultimate...

When they activate this item you should just walk away. They have opted to stun themselves, meaning they cant follow you. If you just walk out of the AoE then they have not only wasted their ult but also their zhonya's active.

Arn't you stunned during his ultimate anyway? Just kidding... but half of the reason to get Zhonyas is with skills like this. I don't see a problem with it personally, because they can't move during it.