How do i prevent ransomware on snapshots

So recently a client got ransomware and had to pay a lot of money. Now we're trying to setup a qnap nas to backup their server files every day. How do i prevent it from backing up already malicious files?. Also if someone leaves their computers on can I still get a snapshot of the email file even if its open or force close them)

Maybe manually turning it off when its not backuping stuff?