Protecting backup to B2 There’s an immutability feature coming (likely in the 7.2 version) that will lock the data for the duration specified in the GFS retention settings. So, you’ll need to use the new backup format in order to leverage immutability.
If you were using our Managed Backup, there are ways to disable the agent and lock it down with a master password. One thing you probably can do is run a backup to B2, object lock the data, change buckets and start backing up to an entirely different bucket to ensure the older backup is safe. You could also back up to an external HD and take it off site.