People thinking “It’s no big deal, they mean no harm” don’t see the big picture here: if they do that, they can do more, or some others can.
I remember in another game, there was the same exploit possible on chat. And moderators could just warn people to stop, and mute them if they continued (no ban allowed for such a “trivial” thing).
But then it all went bad when people played even more with the chat-modification possibilities and killed the chat. They’d type something to make the chat just die: nobody could write anything on the chat anymore. In any chats: guild, whispers, party, shout… and the only way to correct that was to relog.
… It became tiring REAL fast.
(To that you add the usual people impersonating GM and such to try to scam people, of course~)
But banning people for that won’t solve anything either, IMC. Banning is useless if you don’t change how it works. You may ban this guy and maybe others for it, but it won’t stop others from discovering how to do the same.
It’s better to deal with the issue at the source and find a way to make it impossible (or way less easier) to alter the chat like that.