Bitrix24 calendar owner can set various access permissions to different users inside calendar settings. To open settings please navigate to Calendars:
In My Calendars list select calendar you'd like to configure, open calendar options & click on Edit:
Scroll down to Access permissions section and click Add.
For example, if you select Access denied for personal calendars for all users, they won't be able to use their personal calendars and will get the error message instead: