Just setup a read only secret. This is how Bittorrent Sync manages what you are wanting; a one-way synchronization or backup that isn't affected as files are deleted on what you would consider the client side. Folder setup with a read-only secret will be fully synced to the server, but any changes made in the folder on the client will never be synced back to the server. Keep in mind that you can't manage secrets for the folder if access to the folder is managed by a read-only secret.