What does Edublogs use to prevent and stop spam comments

So I am working on an education site, its multisite, and I am worried about spam. I will have captchas and all that fun stuff on the signup and be using the splog api. My concern is the comment spam. While at first Aksimet will be fine, it will eventually grow and become quite expensive to use. Is there something free out there that will work within my network only? Akismet also has a lot of false positives because some students are writing about the presidential debate etc and because it has some term in it, its being marked as spam. Ideas?