There is no way to disable the sync*. You can try to restore the deleted note, but really, the only way to protect yourself from this kind of a scenario is to have proper backups (which synchronisation is not). There is even a plugin for that (see https://discourse.joplinapp.org/t/backup-plugin/13964).
* You could do it in a hacky way by modifying the config file manually and disabling sync there before starting the program.