Forecast and a fixed scheduled top-up are separate authorizations. Disabling Forecast does not cancel a schedule. A schedule buys its approved pack at each due date even when the balance is already high.
Open the scheduled top-up's Change or cancel action or the billing portal and confirm its cancellation there. Changing the pack or frequency requires canceling the existing schedule and approving a new one. If a payment began after confirmed cancellation, send support the schedule reference and both times. An already-started payment needs its own status review.