Unauthorized external user joining Class Team call

On one of our online school days, we had someone asking to join from the lobby with a changed display name with a swear word. We do not allow guest access on our platform. We do not have a log as the person was not admitted into the meeting. However everyone saw the message " so and so" is waiting in the lobby. also next to the name "External" appears.

How can we stop this happening again? what should I advice our admin to set up as policies?