Well first of all, the spammers in general are not RMT, they are account hijackers. They make THEIR money from simply stealing your account and existing gil. Then they place a bot to shout their trap links to get more accounts to shout on. This includes adding random numbers to the end if they want to have unique shouts every time.

I don't think you can actually code a system to prevent them, since they'll just code around it. A GM or something being present and banning in starting regions would be better.