When an account exceeds its burst pack minutes quota, and no more minutes remain in the account, the following actions occur:
As long as the room still has a number of attendees in excess of the soft limit, then new users will not be allowed to join. Existing users will still stay in the room. Once existing users leave, they will not be allowed to join.
Once the number of attendees in the room drops below the soft limit, then new users will be allowed to join again as long as room attendance remains below the soft limit.