I have the same problem on my servers. We get a tremendous amount of spam even though spamassassin is turned on. I wish there was a better howto with regards to configuring spamassiassin to work with directadmin. I also wish there was a gude to show us how to enable per-user contribution to a baysean spamblocking system.
Love direcadmin, but i find configuring the spam system is a bit complex and cumbersome.