USER_IT is not part of Q2A. So you will first need to develop your own permission system that will support the USER_IT permission and then limit access to the categories based on it.
Once you have developed your custom permission system, you will have to develop a plugin to limit access to users without that permission. You can do that using a layer. So each time you display a question you just remove the content or shown an error or do whatever you want.
Regarding question lists... that is the main issue. There is no way to properly remove a question without hacking the core. Maybe you could just display the question in the list removing the title and adding instead a "You don't have access to see this question" message. You can do that with a layer, again.
Related post: http://www.question2answer.org/qa/54987
Plugin layers docs: http://docs.question2answer.org/plugins/layers/