I too delete & ban a spammer about once a week, so that makes 2 x 5 messages per week
So far all the spam is entered manually (I put in safeguards against the automated scripts), so a captcha or the like is not going to work. There is already a limitation on the number of posts a new user can make, which helps in limiting the spam messages.
I will look into seeing if I can make it so that certain email domain users need to be authorized before they can post, but I'm not too sure if the software allows that.