Forum and Sub forums question

[Yago]

On my site, I have a Forum called "Marriage Family and Religion" Now this has a sub forums called:
Marriage
Family
Religion
My question is how do I stop people from posting on the parent forum. I want them to post only in the subforums so that if you have a marriage topic, you post in Marriage and Family in Family. But because the Post New Thread icon is below this subforums in the main forum, some people still post on the main forum instead of the subforums.
You can see exactly what I mean at http://www.nigerians.us
It is a new site about 20 days old so I usually clean up the thread and move it to the right place but as the site grows, this might pose a problem