You can prevent or reduce the chances of a third party attending the meeting in the following ways:
1. By changing the URL (meeting ID) issued for each meeting, you can not join with the previously used meeting ID.
2. Lock the meeting after all meeting participants have joined.
3. Make the meeting available only to those who sign in as a user with a specific domain.
4. Enable the waiting room function.

Q/A

Is it necessary to review the WAN and LAN when using Zoom?
Can I always hold a meeting with the same meeting ID?
The free license disconnects the meeting in 40 minutes. At what point does the time count start?
Can I join a meeting without installing the application?
Can I set up a proxy connection from the Zoom app?
Is it possible to limit the sending of files via chat during a meeting to the organizer only?
Will chat data be logged during the meeting?
If I’m calling from a country that doesn’t have a global dial-in number, which number should I call?