How to remove 'None' from forum options? In D7 advance forum, when users want to create a forum thread at node/add/forum, they are presented with a dorp-down menu fo froum whose default is set to . 

So when users do not choose an existing forum from the menu, their thread is just added to 'None' forum, which is so bad. I need frequenty to remove these 'none' forum threads. 

So I'm wondering how remove this  option or otherwise solve this problem?
The forum is set as a taxonomy reference in the field list of the content type 'forum post'. 

Can you not just set it to be a compulsory field at /admin/structure/types/manage/forum

This way if somebody entered a blank value it would reject the answer and ask them to resubmit, which works better than the default being one of the forums, in which case lots of topics still end up in the wrong place.

The only thing I'm confused by is that I thought this was the default setting for Drupal's inbuilt forum content type anyway? Buy maybe I'm wrong on that.