Additional features, settings, or issues not covered by specific Microsoft Teams categories
Dear Sascha,
If you want to block all Apps in general, in Org-wide app setting, you should set Off "Allow third-party apps", after to set it up all policies, in same Teams admin center, you have to go to Users > in the users list, find the user that you want to assign that policies and click in "View Policies", under "App permission policy", check out if has "Global (Org-wide default)" if is not, change it and save.
All changes made in admin Teams portal, could have more than 48h of replication.
By the way, is not recommended to change the "Global" policy, this policy is default to all users, so, it will affect all users in tenant. You can create a new one, and them assign it to an specific users.
See more information in below link:
https://docs.microsoft.com/en-US/microsoftteams...
Regards,
Renzo.