Hi @Severson, Chris ,
As far as I know it is currently not possible to achieve this, there's a feedback suggestion for this which you can vote for over at the Microsoft Team's uservoice page:
https://microsoftteams.uservoice.com/forums/555103-public/suggestions/40409575-disable-the-chat-function-when-external-users-join
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(If the reply was helpful please don't forget to upvote or accept as answer, thank you)
Best regards,
Leon