There isn't a way to turn off automatic renewal for Ghost(Pro) plans. You need to have an active subscription for your site and account to keep your site online.

Once your plan is canceled, everything is stopped immediately and your data is erased–so we leave it up to you on when you'd like to take your site offline. This also ensures that your data is kept intact and protected. If you'd like to cancel your Ghost(Pro) account and subscription, we have guidance on this here.