To be clear I did not mean it as a dig on your service! It's a really cool idea, sorry for not saying it before. It was just a perfect irony.
You could also do some heuristics like not accepting emails with excessive HTML. Though I'm not sure how much it'd work, since you already require confirmation so presumably the spammers actually have someone authorizing the publication. (I didn't test your flow since I have nothing that warrants posting to say)
