Disabling external chats in Teams for specific users in the tenancy

Hello Christopher_Bell 

welcome to the Microsoft community, My name is Recep I'll be happy to help you today.

I understand that you want to Disable external chats in Teams for specific users in the tenant.

I think you can control this on a per-user basis via the ExternalAccessPolicy controls.

Each external access option has both an organization setting and user policies. The organization settings apply to your entire organization. User policies determine which users can use the options that you've configured at the organization level. Configure the organization settings to specify which types of external meetings and chat you want to allow, and then configure user policies for the users who should have access to these features.

Reference article: https://learn.microsoft.com/en-us/microsoftteams/manage-external-access#limit-external-access-to-specific-people