I usually prefer Google’s standard Android applications over alternative applications, and I would rather uninstall those alternative applications, but deleting preinstalled apps isn’t possible in most cases.
But what you can do, is disable them and also their notifications.
To do this, head to
- Settings > Apps > Apps > Calendar (be sure, you select the right icon!) > Notifications > Allow notifications. Disable them!
and also go to
- Settings > Apps > Apps > Calendar > Permissions. Disable everything!