Leaving a universal is as simple as leaving a regular channel in your Microsoft Teams tenant.
Find your universal channel in your sidebar, click on the · · · next to its name, and select Delete this channel.
This will delete the channel in your Team, but it will not delete the channel or stop the syncing of messages for other contacts in the 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.