If a player makes the same exact statement in /shout, /yell, or /tell more than 3 times in a minute. How about you just boot them out. And if it happens multiple times, then lock them out and investigate them.
I know guilds spam in a similar manner to get new members. But if they're sending 3 spams a minute maybe they should be warned too.
Most the spammers I've seen send 1 message every few seconds.