I've found this useful plugin :
that allows to set specific permission levels to categories.
So who has not the permission cannot see the questions belonging to those categories nor he can browse them.
However I see that he can pick the category for which he has not the permission (the feature you are looking for....), and he can ask a question using that category (bug ?)
I think this is an improvement (fix) that should be added to that plugin, because the current behaviour is not correct.