How to remove a Google calendar from my iPhone

According to my iPhone, Calendar is taking c. 30GB, so I thought I would remove the two GMail calendars, as there should be no dates in them.

If I go into Settings| Calendar | Accounts, and try to disable the calendars, I just get the spinning "thingy", and nothing happens (true of both accounts.

I have been on the web and deleted both calendars from Google, but the same issue persists on the phone.

So, I tried to delete the email account (and would then reinstall with just the emails), but even after c. 2 hours, the account says "Deleting" and nothing else happens.

I have deleted the Calendar app, but it doesn't delete the data, so I still have c. 30GB occupied by Calendar - with the app deleted.

On my iPad, with the same accounts, the same Calendars take c. 3MB!

I have rebooted the phone several times, but it has no effect on being able to delete the calendars.

How may I get rid of the calendars?

Many thanks,