How to remove 'None' from forum options?
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.