Thanks Mike Armstrong for the assist on this.
Under System Properties, I added a new property called jive.usercontainer.discussion.enabled and set the value to false. It would be per content type, so people can still have personal documents but not accidentally post conversations outside of a space or group.
jive.usercontainer.discussion.enabled = false
Now when I try, it says the space field is required and prompts the user to select a space.