Windows Security. Allow user access only to subfolder and not root

I've tried to search about this there and here, but hadn't found a solution.

SITUATION
I have a following folders structure:
root
----|A
----|B
--------|B1
--------|B2
----|C
I need to give permission for user to access only folder B2 and deny access to any root folders. I want to lock user inside this folder.

WORKAROUND
After reading this, I almost achieved it doing these two steps:

1. I gave the only "Traverse Folder/Execute File" permission for desired user to root folder and forced this permission to all subfolders.
2. I disabled inheritance for folder B2 and gave "Full control" permission for desired user.

In this way user can't access root folder's content, but can access B2 folder directly through direct link or shortcut.

PROBLEM
The problem is that user can get one directory "up" from his folder and see root folder B with subfolder B1 along with B2. He can even go inside B1 directory and see all files inside. Although he can't access main root folder root - Windows shows "Access denied" error. Windows Security was always very messy but now I'm out of ideas.

Thank you, TairikuOkami.

PROBLEM
It works in your case, but at the time I didn't realize that my folders are CIFS shares in NAS (Network Access Storage). NAS has UNIX-like OS so looks like the file system and security descriptors are entirely different from Windows. And in my case with your solution user can't access subfolder B2 directly - I get Access is denied.

WORKAROUND
I can achieve your result in my case with "Traverse Folder/Execute File" security option for root and all subfolders enabled, as I described in my first post. But in this case user can't access only root folder's contents. All other subfolders in different level can be accessed. It's interesting, that user still can access the B folder even if I disable inheritance of B and entirely remove user from the list.

CONCLUSION
NAS and Windows Security systems between themselves do not work properly. At this point I'm curious how they managed this to work at all... :)