Q: How do I stop a multipass from renewing using my iPhone?

View, change, or cancel your subscriptions - Apple Support (doesn't look like it applies to multi-pass)

Hi

Go to itunes bottom of page Apple ID

Click on that go to manage subscriptions turn the ones off

You don't want to renew.

Cheers

Brian (uk)

I know you're not trying to change your country, but this article, about changing your country, seems to imply you cannot turn off a Multi-Pass:  If you can’t change your iTunes Store country or region - Apple Support

The important part is:

"You must wait for the pass to be completed. When there isn't anything under Current Passes, you can change the country or region of your account."

If it's that important, I would contact Apple and see what they can do:  https://www.apple.com/support/itunes/contact/