There is an article in the wiki that tells you how to improve the anti-spam system if you'd care to read that, there are also threads in the forums that details what you can do to stop things like backscatter and improve your anti-spam with things like sanesecurity - have a search for them.
It might also help if you posted details of what you've tried so far and what some of the headers are from the spam you're receiving.
BTW, you will never stop 100% of the spam coming into your server so you'll have to resign yourself to that fact.
You should also update your
forum profile with the output of the following command (do not post the output in this thread):