(New) Can you stop a specific user from reading po

I’ve seen that, and it work in the backend too. But when you have several users/usergroups to deal with, and several posts each of them with differents accessibility policies, it’s starting to be a nightmare to handle dozens of passwords.
Is there a way to tell WP, post by post, and/or categories by categories:
– this post/categorie is (or is not) readable by default
– these users override the default, and thus can (or can not) access this post (or the posts within the categorie) even in the backend
?

I’ve overlooked the ViewLever plugin, but I don’t know is it’s able to do this.
Nope, no ban or not public 🙂 It’s for a CMS usage: a community site, maybe 20 users (to start with), all of them have to write post from time to time. One or two of them are admin, able to do whatever they want; but the others have to be able to read (from the frontend or the backend) only specific posts and/or categories, and not be able to read others specifics posts|categories.
How do I put that…ok let’s take a company intra/extra-net/webiste for example: you have the customers, who just read public posts. But you have ancient, or “advanced” customers who can wrote posts to help others so they have backend access, but they can’t be allowed to access in any way specific posts/categories even in the backend. And you have company employees… you don’t want a C++ intern developper newbie to have access to internal marketing posts, or accounting posts…and so on.
See what I mean ?
Sorry for the laborious explanation, I’m not english speaking native.
Si, what I need is a way to say:
– this post (and/or category) can’t be read by the users x, y and z; even in the backend.
and even better:
– this post can’t be access at all, except for the users a, b and c.
and so on, and so on, and so on.
Really alike any modern right management… like in unix, or MySQL, and the like.
Can I do that with WP ? Or a plugin ? Or a hack ?