I never hosted a forum, so i have no idea how easy it would be to accomplish one of two possible solutions:
* The mail informing me could give me the first 5 lines or so of the new post, which is mostly enough to spot spam.
* Some of the spam is so bad it might be a bot - so captchas might work - I hate them as much as the next person, but there is a version that at least has some good angles to it :ReCaptcha- the captchas are words from a digitalization initiative, that the OCR could not cope with. By solving the captcha, you help digitize Books! Yay! (It still sucks, but with a nicer aftertaste
