I understand that you've paid for someone else to upgrade their iCloud storage, and you'd like to stop this. I'm happy to provide some help for this.
To pay for iCloud storage, you would have either needed to buy them an iTunes gift card, or added your credit or debit card to their account. Since it sounds like you are seeing the payments on your end, it's more likely that you've added your payment information to their account. In that case, you will not be able to stop these payments without their help. The following article shows how to downgrade the iCloud storage:
Downgrade your iCloud storage - Apple Support
And this article shows how to change the payment information:
Change or remove your Apple ID payment information - Apple Support
If you're unable to do this for any reason, contact your bank or card provider for additional help.
Thanks for using the Apple Support Communities. Cheers!