At this point spam is not really an issue. I currently manually moderate all comments that come into the system. Obviously this isn't going to scale for too much longer but it was the quickest way to move forward. In the long term when spam becomes more of an issue (because t inevitably will) I'm planning to allow paying users to input an Akismet API key.

If we grow out of that I can implement my own bayesian database to to learn to filter spam.

The sky is the limit in cat and mouse. I'm just waiting for the mouse to become smarter then what I currently have in place.

The other option is to prevent comments from showing up until it has been explicitly approved.  That option is already in place for paying users.