Go into iCloud settings and disable the iCloud Backup setting on each iDevice. Then go into backups and delete one of the backups. Might not be what you want to hear but that's the reason why you keep getting that prompt and the only way to remove it.
Alternatively, if you want to keep a backup with 5 GB of storage you can back up your iPhone and iPad but stop saving documents to your iCloud Storage. This also means not using iCloud Photo Library.