I downgraded my icloud subscription to just 5GB, but I still receive notifications that I don't have enough balance to complete my payment. How do I stop apple from charging me for my icloud?

Seems if you downgrade or cancel your iCloud+ plan, the change takes effect after your current subscription billing period ends. (How to: Downgrade or cancel your iCloud+ plan - Apple Support)

Check whether your subscription plan did actually change to the free one when you downgraded:

(You may need to be sure your iCloud files take up no more than 5Gb space.)

(You can Check your iCloud storage on any device - Apple Support)

If needed:

Try contacting iCloud Support (via online chat / phone / email etc.) from the "Get Support" link on their webpage:

iCloud Support website: iCloud - Official Apple Support

or

Billing and Subscriptions Support Website: Subscriptions and Billing - Official Apple Support