If the Centera is in Compliance Mode or Compliance+ mode, then you won't be able to delete anything until the retention period has expired. If those Archives are eligible for deletion, running Storage Expiry from the site settings will usually delete the archives. If they still don't disappear after running, expiry, restart the storage service as well.
Now, one thing to keep in mind is running Storage Expiry will expire all items older than it's retention so this is something to be careful with if you don't want to expire items in active archives.