Well at the very least, there are common traits among all spammers. You could hash the message in every note they post and come out with the same string. Spammers (unless they have to in order to get around this filtering heuristic) tend to incessantly post the exact message word for word post after post.

You might catch "good morning" posts using this heuristic, but a little tweaking and I think it could be a good place to start.