Thanks for posting here and welcome to the community
I’m not sure I understand the limitation for unsyncing the calendar. Are you not able to do this from these instructions?
How to change the Google Calendar linked to my account?
You will need to un-sync the current calendar and then sync the new one. Please follow these steps.
Click here to access your Google App settings.
Under “Apps with account access” click “Manage Apps”.
Choose monday.com and click “Remove Access”.
Next, log back in to your monday.com account and follow the steps listed above to sync your Date or Timeline columns with the new Google Calendar.
You will be prompted to choose the correct Google account to link. Once you sign in to the correct one, you should be all set!