Completely remove any potential for subscription renewal

I've recently accidentally subscribed to a 1 year plan for a piece of software. The subscription would start after my 3 day free trial period.

Within settings / apple ID / subscriptions, I have removed the subscription, but my phone still says "your ... subscription expires on 11 September".

I don't want the software, the software didn't work for me and I would never have paid the author for such a pile of nonsense, anyway.

How can I guarantee that Apple never take any money from my account?

Thanks.