When you, or your customers subscribe to an ECA calendar, they might do this through:
- Apple Calendar (on OSX or iOS)
- Google Calendar
- Outlook Calendar
The precise steps to unsubscribe will depend on the platform you or they subscribed with in the first place.
Here is some links to the documentation from the main calendar providers on how to unsubscribe
Apple
https://support.apple.com/en-gb/HT202361
Google
https://support.google.com/calendar/answer/37188?co=GENIE.Platform%3DDesktop&hl=en
Outlook
https://support.office.com/en-us/article/delete-a-calendar-9ba1364c-bdb0-41be-8f66-eaf96698d021