Before you start
This only applies if you already installed a contact for external direct messaging as listed out here.
Removing or uninstalling an external user (app) from Microsoft Teams
In your Microsoft Teams app, click on the Apps icon in the bottom left corner.
Find the app of the user your want to remove. If you don't see it on the first page, click See all under Built for your org.
Click the - - - next to their name and select Delete.
Check the disclaimer message and select Delete app.
You may have to wait a minute for the change to occur on your screen. Try navigating away from the Apps screen and checking back in a minute to confirm that the user's app has been deleted.