Hello Gjones-uk! My name is Jeremy and I’m happy to help you with your question.
The ability to close a large number of chats is not currently available in teams.
I can understand your viewpoint on this feature however and would recommend sending feedback directly through the application.
Here is a link on how to send feedback directly to the development team about possible future options or QOL changes.
https://support.microsoft.com/help/c0fb6297-22af-4db5-b19b-69e0a6720927
Thank you for reaching out to the community forums and have a good rest of your day.
Regards,