Applications that are connected to Slack are managed in the App directory. You can uninstall MiiTel from Slack by removing it from here.
How to uninstall.
- Go to the Slack App directory (https://my.slack.com/apps).
- Click Manage in the top right corner of the screen.
- Enter "MiiTel" in the "Title Contains" search box.
- Click on "MiiTel" in the search results.
- Click the "Remove App" button.