As in my example, spamming the shout channel with a conversation is already, in my opinion, impolite to the other players that legitimately require to use said channel.
While back-and-forths between two people which don't in any way concern anyone else should really be taken to /tell, general chatting amongst many people in a zone is not "rude" and should not even be discouraged. The chat channel is for anyone to use for any purpose and you have no right to decide which purpose (or whose purpose0 is more important.