It's possible the lock out the conference room from new attendees. Also as someone who drives conference calls, I would be sure to eject anyone who I did not know on the call... I monitor participants constantly.
I use zoom for interviews and run into the same issue using my personal room for back-to-back interviews. My solution is to enable the waiting room feature. This makes it so that people can call in, but need to be admitted to the actual room. This works well for the interviews. The problem is that it's a account wide setting and cannot be disabled for specific meetings. So now the is a problem of I set up the room for a recurring meeting and am out sick and forgot to disable the waiting room.
Yes, that would be a potential solution, but using the same room works exceptionally well with how the meetings get scheduled without going into too much detail.
I see two issues with that strategy. If you lock a meeting, people can't connect who may just be late, or if they disconnect accidentally, they can't reconnect. The second is most calls aren't webinars or organized events. These services get used by 1-on-1, 2-on-1 etc, smaller calls frequently. It could be difficult to stay engaged and be expected to diligently secure a call.
I don't think we'd expect this level of security to be acceptable for email access. It's really just a matter of enforcing a pin number, along with a meeting code.