How do I get Apple to stop telling me that my iCloud storage is full and subsequently trying to get me to buy more storage?

Apple is not trying to sell you more iCloud storage. The message is suggesting that you buy more storage and will continue to pop up unless you delete some content in iCloud or buy more storage.

What you decide to do is up to you.

Did you see the message about not being able to upload photos, files and messages to iCloud because you don’t have enough storage? You are getting that message for your own good. It is a warning. If you like, just ignore it because you can’t get rid of it.