I just looked this up, and for some reason, Teams is a category to use in Layer 7 Deny rules in the MX firewall settings (under VoIP/VideoConferencing), but isn't an option when updating a Group Policy object. Very odd. https://docs.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-worldwide This is the page that lists all of the FQDN and IP's used for O365 services, including Teams. You can use Group Policy to block the IP ranges for Teams. In your Group Policy Object, I would suggest testing a Custom Layer 7 Deny rule that blocks traffic to the Remote IP ranges needed for Teams use. These seem to be the IP ranges needed for Teams use: 13.107.64.0/18, 52.112.0.0/14, 52.120.0.0/14, 52.238.119.141/32, 52.244.160.207/32, Hope this helps...
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