I had this problem a few years ago and it beat me!
I had intended to launch the forum AFTER I had built a good following at a particular blog but in a "moment of madness" I launched it anyway.
The main issue really is that you have no help - and all good forums have a load of moderators to share the work with you.
I looked for some kind of phpbb version of akismet to no avail. I tried blocking I.P. addresses, I tried all kinds of stuff but in the end the spam ( all of which was **** which made all the more offensive to me) beat me. I locked the forum for a while and then I deleted it.
In the meantime you could just build a following at twitter and put a wordpress blog on the domain too if you don't have one yet.
Build a list, recruit some mods and good luck!
Alex