Multiple Screens For same User!

Hi,

I have a server set up on Ubuntu 18.04 LTS and have just been testing my room and password.
With 4 in the room I noticed one was occasionally freezing. On checking his bandwidth was about one eight of the rest of us (240Mbits).
I asked him to reduce video bandwidth but that made no difference to the freezing.
The worrying problem was that every time he logged back in it created another window with the others frozen and disconnected!
Even when we all vacated the room and logged back in, the screens were still there and I had to switch off the server to loose them!
I don’t want to have do this in the middle of a meeting.
Can anyone advise on why this is happening and any suggestions on how to stop it happening and if possible, is there a way as moderator to delete users without rebooting the server?
PS I have asked him to check his internet speed/connection with his ISP!

Thanks

Peter H