We need a restriction on registration or the ability to send messages for new accounts, clearly the 6-minute delay for newbie is not enough.
If someone wants to post more frequent, he/she will register a new account.
If he/she has 2 accounts, it's 3 minutes...
If he/she has 3 accounts, it's 2 minutes...
If he/she has 6 accounts, it's 1 minute to wait if he/she wants to post with both accounts continously...
If he/she has more than 6 accounts, it's physically not possible to post that much, unless he/she has a bot to post....