You can have a blacklist of message texts that are sent to the apps as hashes

Good idea! That could definitely work.

This seems like it could only be implemented client-side. It also seems like it would require rapid and frequent updates to the ever-growing blacklist. That in turn would require a way to figure out what hashes needed to be blacklisted, which implies either less-than-ideal cryptography practices or the ability to decrypt messages. Neither is desirable for user privacy.

I have a rule I use in computing: whenever you think you want a blacklist, you probably want a whitelist.

Well these fake viral messages are usually quite temporary so you could expire the cache after some time and if some messages have particularly long “legs” just re-send them to clients after their TTL expires

Well done! You've elegantly solved the problem of the cache being append-only.