The 'hang up' button has always been a way to end audio and video in a meeting, but you remain connected to sharing and IM. This is a feature that customers use to continue monitoring a meeting without remaining connected. Note that if you close the meeting window, you are fully disconnected from all modalities of the meeting.
If you can't close a conversation window, that is a serious bug that needs to be fixed. We have not heard from users of this issue. How often do you run into this? If you are using the insider build and encounter this, please report the issue to us by clicking Help/Report an Issue.