Doesn't filtering user content jeopardize freedom of speech?
What if someone wants to have a legitimate discussion about Viagra or pornography? Those are words that can't be freely used in emails because today's email spam filters would likely block them. In other words, email anti-spam systems have effectively undermined free speech -- I cannot express myself freely without wondering if certain words are going to trigger a filter and prevent my message from being delivered to the recipients' Inboxes.
Mollom's spam blocking occurs in two steps. Only the hard-core spam (of which Mollom knows for sure that it is spam) will directly be blocked. All others posts which Mollom thinks are possibly spam, will be marked as unsure. This means that Mollom cannot be 100% sure that it is spam or not. In those cases the user will have to fill in a CAPTCHA to authenticate being a real human before the content can be accepted. More on this can be found in the How Mollom works section.
So if your users want to have a discussion about Viagra, they are free to do so, but might have to authenticate their human-ness. So Mollom fully allows free speech, between humans :)