How to stop nextcloud calendar and stop syncing calendar items

We are running nextcloud for a couple of years and also the Calendar options, so users can sync and share their calender to other users.
But now we just want to stop this feature for users. The point is, they have a new Calendar on some other platform.
But the calender keeps syncing and users can still use the feature, we cannot figure out how to stop nextcloud from doing this.
We disabled and removed the Calendar app in the admin mode, but that will not stop the nextcloud ical calendar… Users cannot use via the nextcloud web interface this feature, but with their clients (like Apple calendar) they are still able to use their Calender and create appointments and share these…
Is there a way to stop it forever?

You might be able to achieve this with Calendar / CalDAV — Nextcloud latest Administration Manual latest documentation

Or a firewall filter.