Well there you go, a registry setting to the rescue.
As for the ransomware/hacker scenario though, disabling the notifications doesn't really change anything there. A program that tries to encrypt Reflect files with Image Guardian will see errors regardless of whether the popup is enabled. You can try it yourself by disabling the popups and then using something like Command Prompt to try modify a file; you'll still get an error. In addition, the popup notifications would only come into play for someone who's logged in interactively, which ransomware "distributors" won't be. They're going for maximum infection and maximum efficiency, so they just want their application to run on victims' PCs and send back a unique ID and a decryption key for each victim so the hacker can then extort the victims. They don't care about watching anything happen on the victims' PCs. And even in an edge case where a hacker somehow had malware that gave them remote viewing capabilities, seeing the popup notification doesn't give them the opportunity to do anything about it. They'd still need admin access to disable Image Guardian, and if the hacker has that, then frankly it's game over anyway. The most common threat model, and the one Image Guardian is meant to protect against, is that a victim ends up running a malicious application that runs with non-admin
rights and encrypts files it can access, typically in the user's profile folders (Desktop, Documents, etc), external hard drives, and network shares -- since files in those areas can be modified without elevated privileges. That's partly why ransomware is so dangerous -- because it doesn't NEED to have a privilege escalation vulnerability available to exploit. It can cause problems even for people running as standard users. That's where Image Guardian comes into play.
As for the rest, I get your point that the users you support probably prefer not to think about backups themselves, but if they're not doing anything with their backups, then they shouldn't be encountering that popup. And if they are, then once again I'd have thought they'd want to have a clue as to what might be preventing them from doing whatever they're trying to do rather than just getting a somewhat opaque "storage policy" error that doesn't mention Image Guardian at all, but if it's preferable in your use case for users not to be aware of what's going on, then it looks like Macrium already has you covered