Primary Google calendar connected to strange account even after deleting

Hi all, I'm at a complete loss here and hope that someone may have been in a similar situation and is able to help me out.

My primary google calendar is getting shared with another person (under "Share with specific people or groups"). When I remove this entry and refresh the site, it disappears. When I change their visibility to "event status only", a new @gmail.com email appears in the list, that apparently belongs to the other, unwanted email (at another provider). I just tried deleting the entire calendar, and it worked, my events are gone - however, a new primary calendar was created which, you guessed it, is again being shared with this account!

The account belongs to a family member that I certainly don't want to have access to all of my personal/work events!!

EDIT: I exported all my events to a new calendar and renamed the primary calendar to "DO NOT ADD". However I can't even hide the calendar from hell as it doesn't seem to be possible to change your primary calendar.