I don`t think you can stop it entirely Greg. There is someone out there always figuring out how to get through the back door. It has gotten a lot better though, and you probably see the spam a lot more than the users on the site. I personally haven`t been bothered with it at all except for that one time right before you did the passcode and msg limits.
My wife has a retail website and she still gets spam sent to her, even with filters put on the mail servers and at the hosting sites. Its part of being a web owner unfortunately. But if you figure something out, let me know so we can use it on her site as well! :)