Shoutbox

If the error is only written in the conversation window, I see no reason to have Plus! do the limiting itself. Being flooded with that kind of messages is exactly the same as being flooded with normal messages from a contact. It can happen to anyone, even without Plus!.
Furthermore, in this case you can enable auto-accept for trusted parties only, while text floods can come from anyone on your list (or even from people not on your list if your security settings are loose).

Getting pop-ups from it is a different thing though.