Apple offers two solutions to this problem in Share iCloud+ with your family but neither are what you want to achieve if the member is to remain in your group:
Buy more storage space, or
manage the storage on the family member's device and iCloud to free up space.
You will find this under the heading 'Ask family member to increase the iCloud+ storage plan'.
I would query your assumption that your family group does not need more iCloud space because you are already asking how to manage an inadequate amount. Perhaps you could get a contribution from the member who is using most of your combined allowance to acquire a larger plan?