How to prevent external users from reading Teams meeting chat after they leave the meeting?

ThePuh Well, you can end a Teams meeting for everyone in case you're not familiar with that, but it doesn't disable the ability to rejoin so Adams suggestion of taking sensitive notes somewhere else makes sense.

https://support.microsoft.com/en-us/office/end-a-teams-meeting-for-everyone-in-attendance-5051fa72-8470-4a38-99e4-014535208971

But you can use the above procedure and in combination with a Teams meeting policy (or using the meeting options for an individual meeting) and set the Lobby to "Only me" (for example) so that everyone would need to be let in by you again.

-AutoAdmittedUsers

OrganizerOnly, if you would like that only meeting organizers can bypass the lobby.

https://docs.microsoft.com/en-us/powershell/module/skype/set-csteamsmeetingpolicy?view=skype-ps

Change participant settings for a Teams meeting

https://support.microsoft.com/en-us/office/change-participant-settings-for-a-teams-meeting-53261366-dbd5-45f9-aae9-a70e6354f88e

Haven't tried it but should work.