There is a lot that can be accomplished by using register_on_chat_message. Not only can the server say things back to the player, but you can code things so the message isn't sent if it contains certain words, or results in a kick, or even a ban. It's entirely up to you and your code.
minetest.register_on_chat_message(function(name,message) if string.match(message, 'foobar') then minetest.chat_send_all('<Illuminati> Hush, this is not public information.') end end)
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