You can stop the backup service by clicking the link "Stop the backup service" on the bottom of the Backup Page on the family site, which is found on the family site in the Family Tree tab.
Backups are kept only while the backup service is paid for. If the service is stopped for any reason, it will keep prior backups intact for a grace period of 30 days to allow you to download the backups to your computer. After the 30-day grace period, backups cannot be restored.