Before putting a lot of effort in this, just to find out it's not possible:
Let's say I have two different categories and I want the user to be locked in one of them after choosing it from a menu-point. If someone has choosen, e.g. 'announcements', I want Wordpress to do like there are only posts of this category and ignore the rest. Or even better, is there a way to divide categories into different blocks and use them separately? Would this be a job for link-categories?