Just two limitations that have no effect on anyone who isn't a spambot.
- 100 characters (spaces included).
- Can only send a message in /shout or /yell once every 5 seconds.
No human players have any need to send single messages above that theshhold and that frequently in /shout or /yell.
It will be much easier to blacklist and report spambots when people don't have to try to freeze their chat pane and manually scroll up/down due to a message twice as big as the chat window being spammed literally every single second.
Simple. Effective. Do it.