Microsoft Teams now allow administrator to define policies to pin apps to their end-users (turned on by default).
This policy setting also allows administrator to disable the ‘pin app’ capability.
As remined, an app can be pinned in Teams by following these steps:
For administrators to define/update policy, logon to your Teams administration portal (https://admin.teams.microsoft.com/) and go to Teams apps\Setup Policies, then either edit an existing policy or create a new one
Finally turn off the Allow user pinning if you want to disable the feature or define the apps to be pinned to end-users