Problem with NavMesh

Hey guys, I'm having a really frustrating problem with getting my NavMesh the way I want it, and the problem is that I'm trying to block off an entire section of my map from access, but for some reason, the NavMesh still bakes these areas into the mesh, albeit with chunks cut out to denote the parameter where I set a cube to act as a collider. Am I doing anything wrong?

I'm trying to get the area covered by the white block to be inaccessible.

But all the NavMesh is doing when I try to bake the area is just cutting out an area where the parameter of the block should be, but leaving the insides accessible.

It's not a unique problem. I already checked by creating another project, and it's still doing the same thing. And I already experimented with an AI, and it tries to access the blocked off area, when I randomly pick a walkable location that happens to be inside, but can't reach it because the collider's in the way, even though that area's supposed to be totally cut off. Is there anyway to do what I'm asking at all?