Ever heard of copy/paste?
The only way to fix it is to add a zone wide chat dedicated for spam.
Of course I know some will copy and paste, I'm not ignorant to believe nobody will do so.
The fact that they would have to copy and paste whatever prefix is proposed inconveniences casual conversation. You don't think copying and pasting all the time just to hold a conversation is annoying in the least? I'm not talking about trying to prevent spammers who purposely spam with the same repeated nonsense, because they'll abuse any system available. I'm looking to rid of casual conversations not related to Trade. I wouldn't mind at least some players who want to global chat to move back to !Shout for their spam since it'll be easier in this system, then at least i'd be able to filter the shouts using the chat options and just see the players who want to sell and buy.
Your suggestion, a zone wide chat dedicated for spam, will only add another avenue for spam, not eliminate it from trade or general chat. You're giving spammers a room to spam, but not preventing them from spamming the other Trade or General chats. You can give them a room to spam, doesn't mean they will use it. How is that the "only fix"?
If you have any better ideas, feel free to share.