Leaving a universal is as simple as leaving a regular channel in your Slack workspace.
Find your universal channel in your sidebar, right click on its name, and select Leave channel.
Deactivate your universal channel entirely
If you are the channel owner and want to deactivate your universal channel (prevent messages from flowing to external contacts and other apps)
In a direct message with the Mio app, send "channels".
Select the universal channel you want to deactivate.
Click the Deactivate button at the bottom of the Channel Details message.
If you don't see the Deactivate button, please check who the channel owner is and ask them to deactivate the channel instead.
For more details, check out this article.