wmcs-backups has a config file where you can specify a list of regexes to exclude some volumes from being backed up:
exclude_volumes: cloudinfra-nfs: - scratch admin-monitoring: - ".*" [...]
This is not working correctly because the code in wmcs-backup.py is matching the regex against the image ID (e.g. d1478efd-9fa6-4293-8389-e72459b794c0) instead of the image name (e.g. scratch)