Sorry for the late answer ! People will do what’s easiest, not what’s right. So one way to make them behave like you want is to make it easier to do things right than to do them wrong.
When hitting the “new topic” button you are presented with a form to create a new topic and the category “uncategorized” is already selected and not put forward in the UI. I think leaving that category empty will force users to actually select one before posting, increasing chances of posting in the right category.
The forum code you’re using also allows to give users the ability to move topics to another category. You can grant that ability through a badge to only certain users based on their presence on the forums, it’s very useful because it allows experienced/frequent users to do some small moderation like editing titles and moving topics.