Well, it had just crossed my mind that there used to be a text file created within the Graal folder called "mutedaccounts.txt" at one point. I must not have muted anyone since installing Graal on this pc though so I never gave it another though, but having muted an account just now, the file appears to still get created.
If all else fails, I would give deleting the text file named "mutedaccounts.txt" in your Graal folder a go and seeing if that helps achieve what you're trying to do.