You really should learn to read. You can start by learning to read the title of this topic, then maybe you'll learn what this topic is about. There is no need for you to tell people what you think the topic should be about (especially when its exactly what I have already said), unlike you they already know. They read the title.
You're talking about blocking walls from anyone being able to go over them at all. This would significantly reduce the resourcefulness that champions with blink and jump abilities have.
From what I can see, your solution hurts way more champions than just Kassadin. It has a lot of collateral damage.
Tristana, Fiddlesticks, Corki, Leblanc, Kassadin, Ezreal, Sejuani, Shen, Vi, Shaco, Katarina, Leona, Lee Sin, Jarvin, Diana, and Amumu would be significantly effected with your idea. And that's just off the top of my head. Yes, it gets Kassadin, but also hurts champions that are already in need of help.
This is what I see from the original post. That's how I took it.
So, to make it simple and to only effect Kassadin, I suggested we nerf his jump range. The only champion effected would be Kassadin. It's not that hard to understand.