If no one is using nextcloud. WARNING you may delete data: You can enable administrator login in nextcloud and either disable user by user and test
or go to >> Administrator Settings >> Basic Settings >> Email Server >>> Sendmail
JUST change to some nonexistent username or password and see if it stops sending notifications.
As a last resort just delete the users.
You are own your own this is unsupported and you have to run the script to enable the admin username in MIAB.
Also there are some more elegant ways to purge the calendars as per this: