We are working on providing the ability to remove external users from a universal channel. You can check on the status of this feature by going to our roadmap at m.io/roadmap.
Right now the only option is to "deactivate" your channel, which will remove the connection with all external parties.
How to deactivate your universal channel
Open the Mio menu by sending the word "menu" in a direct message with the Mio bot. Select "Account" to log in to your Mio account using the platform you installed the app on.
On the right side, select Channels to view a list of all your universal channels.
Click the · · · under the actions column and select Deactivate.