So, do you mean the user not able to access a page (or pages) changes? It's not always the same user?
Users X,Y,Z can access pages A,B,C but not User W
Users W,X,Y can access pages D,E,F but not User Z
That does make it more complicated but still possible using user groups and categories together. I think I need to understand exactly what you're trying to do, in order to give a better solution.
Another possibility that I just remembered is that you are able to create one user group per user (at registration time), so then you have to manually add each user group to each page that they have permission to see. This is a lot more (maintenance) work but obviously gives you the greatest flexibility.
It all depends how often the pages and/or users are changing and what combinations of user access are required (and also the number of users/pages we're talking about). If it's always user X that cannot access certain pages, then it's fairly easy to do this. If we're talking about hundreds of users and/or pages then I'm afraid there is no easy solution, it's going to take a lot of maintenance however you choose to manage it.
If you could give some more detailed information that would be helpful.