Spam Blocking

I have been getting spam through one of my sites. At first I thought that it was coming through the comment form at the end of each post. I did close down the un-gated comment form – thought that it would stop the flow of spam – but it did not. Apparently that is a hole in WordPress that allows spammers to send comments without the form – a real problem with this kind of open source stuff. Neither SlimStat nor Akismet gives me any indication of how they are doing it – where the access is. Any ideas as to how I can find the tracks in the snow?