There maybe situations where you want to allow access to Microsoft Teams using the desktop or mobile app only. In the Teams services control settings within the Office 365 portal, you have some varied settings, but nothing that can enforce app only sign in and use. Thinking about adoption and encouraging use within the business, you may want to make your communications simpler, avoiding confusion over whether to use the web client or the desktop app. From experience, being over accessible when first launch often causes a reflex action by the end users. This is because people are comfortable with routines. Disrupt their routine, no matter how trivial unbalances them significantly and their instant reaction is to perform a Fred Flintstone emergency stop!
The art of introducing a new way of working and a new application is to start slowly and ease them into it. It’s all to easy to blow their minds with all the possibilities of what the app can do, but that just outputs a negative adoption result. With this in mind, perhaps you want to launch Microsoft Teams in your business, but want them to use the apps instead of the web client or both for consistency in functionality and training?
Before you go into level 900 technical investigations about how to proxy block https://teams.microsoft.com but still allow the app to sign in and use these URLs there is a simple and easy way to block sign in from the web.
Open Office 365 Admin Portal and then launch Azure AD. Then choose Enterprise Applications from the menu
From the Enterprise Applications menu, select All Applications
From the filter choose Microsoft Applications with state Any and press Apply
Scroll down until you find Microsoft Teams Web Client and then click on it
Now from the Teams Web Client Menu, select Properties
Change the setting for Enabled for User Sign in from YES to NO and press Save
Wait a few minutes for replication and when you try and login to Teams using https://teams.microsoft.com you are blocked
A bit of an unpleasant error message, but in the additional technical information you see the reason
However, I am still able to login to the Microsoft Teams App without any trouble