Here's how to turn it off for good and pay what you originally expected you'd be paying before Microsoft tried to swindle us all for the pleasure of using their AI:
In an Office product go to File - Account - Manage, then Manage Account.
Sign in to your Microsoft account and in the Subscriptions Tab - Manage Subscriptions navigate to the bottom option "Cancel Subscription."
This will take you to an option where you can choose the original plan you subscribed to, Microsoft 365 Personal Classic, before Microsoft tried to swindle you out an extra £25 by upgrading you without your premission.