I don't use sections, but I believe this issue could be solved by making groups that completely overlap with the sections.
So you can make a "Groups Section" that splits the class into groups, composed of the members of each section, and if you select the discussion to be a "Group Discussion", choosing the "Groups Section" as the grouping, they should be able to view only messages inside their groups/sections.
--EDIT--
Indeed, that is the recommendation listed in the blue box of this guide: How do I limit a user to only interact with other ... - Instructure Community - 1211
As limiting users from interacting with users from other sections does not apply to discussions, it recommends using groups:
Discussion topics and Pages are not affected by section limitations and can be viewed by any student. These feature areas could be restricted by creating content in course groups.