You just need to block the partner company domain in the Org-Wide Settings > External Access.
Org-Wide Settings
External Access
External access lets your Teams and Skype for Business users communicate with other users outside of your organization.
Ord-Wide Settings
External access
Add a domain
Note: By default, your organization can communicate with all external domains. If you add blocked domains, all other domains will be allowed but if you add allowed domains, all other domains will be blocked.
Note: By default, your organization can communicate with all external domains.
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