Go to settings / your Apple ID at the top / iCloud / iCloud Backup and turn that off. You'll device will no longer back up to iCloud and keep asking you to buy more storage.
You may also want to disable Photos storage to iCloud also and get a USB drive and back your photos up manually.
You only get 5 gigs of free iCloud storage. Once that limit is reached you can either (1) stop backing everything up to iCloud and doit manually or (2) buy more iCloud storage space.