Following these steps you can "Hide this app and stop selling" from the Windows Store:
How to unpublish an app in the Windows Dev Center
This prevents new users from finding and installing the app, the issue is that current users can still run it.
This has always been a free app and the reason is we're migrating to a web version and the Windows one will be supported no more.
According to the documentation Removing an app from the Store:
Note that any customers who already have the app will still be able to use it (and could even get updates if you submit new packages later).
I'm afraid you're not able to force uninstall for current users.
Since you have been migrated to a web version, you could send a notification (if it's supported for your previous version) to notify current users that the Windows version app won't be supported any more and attach the link of the web version so that the app service can still be available to them.