Question about "Allows users to reply to system messages, even if …"

Context: With our current settings people are replying to system messages (e.g. your post has been flagged) and then they are replying to those system messages, which of course the moderators can’t see.

In settings, there is this:

“Allows users to reply to system messages, even if personal messages are disabled”

Currently we allow personal messages. If we untick this box will all of the reply buttons be removed from messages from the @system user?

It has been suggested to me that

unchecking this means “only allow replies to system messages if PMs are enabled”.

Another solution was suggested:

disable ‘Allow other users to send me personal messages and chat direct messages’ on system’s preferences’ (/u/system/preferences/users)

What is the recommended way to stop people from replying to system messages?