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.

1. Go to the Slack App directory (https://my.slack.com/apps).
2. Click Manage in the top right corner of the screen.

3. Enter "MiiTel" in the "Title Contains" search box.
4. Click on "MiiTel" in the search results.

5. Click the "Remove App" button.