[Phanx]

Without seeing the addon's code I can't provide the exact solution; ideally you would want to keep track of who you tried to message when you got that error, and avoid attempting to message them again.

However, here's a brute force approach that will simply hide the errors, and isn't dependent on anything else in the addon:

local CHAT_IGNORED_S = gsub(CHAT_IGNORED, "%%s", ".+")
ChatFrame_AddMessageEventFilter("CHAT_MSG_SYSTEM", function(_, _, message)
    if strmatch(message, CHAT_IGNORED_S) then
        return true
    end
end)

The addon appears to be using its own "player is ignoring you" message for some reason, as no such global string exists, but the CHAT_IGNORED global can easily be modified to catch it, at least for English clients.