As a preventative measure, Waiting Rooms are enabled by default (Host controls who enters the meeting). Here is what to do if you have an uninvited guest get in a meeting:
1. Lock them out. Go to the Participants List in the navigation sidebar, and scroll down to More. Click Lock Meeting to stop further participants from entering the meeting and to be able to remove participants.
2. Shut them up. Have yourself or one of your co-hosts go to the Participants List, again scrolling down to the bottom, and click Mute All Controls. This makes it so the unwelcome participant can't use their microphone to disrupt your audio.