You can turn off Monthly Top Up for prepaid Nova numbers in the Nova app on your phone.

You simply open the Nova app and log in with a phone number.

Then you select the user icon in the upper right corner, and an overview of your services will appear. Select the phone number you want change.

Next, you need to select the Top Up itself. It shows an overview of usage and if you scroll to the bottom of the page there is a button called "Disable autofill".