From what I can tell, it is only possible to disable the discussion feature globally. Is it possible to turn off this feature for just a single course?
Right now, built-in Discussions can only be switched on/off on a global basis (i.e. not for individual courses). It's definitely something we can look into for the future, though.
For the meantime, you may find that our Disqus integration works better for your needs. More on that here.