It is a big problem with Blogware. I had a huge amount of trouble with it, to the extent it entered the same bracket as washing-up and vaccum cleaning in my level of irritating jobs that had to be done. The best solution is a painful one: move your blog to Wordpress software and install Spam Karma - which is incredible. I have been bothered by maybe five bits of spam and two trackbacks in six months, where before that was a very good day. If you do decide to shift though, ask for advice from those that have been there - you won't regret it. Kieren