There are ways of keeping blog comment spam from getting through but some of the solutions may be more trouble than they are worth. I've talked before about what I've done on this site but this guy has tried experimenting with various other anti-comment spam methods and reports his results.
Although he is using WordPress as his blog software what he has done should be applicable to the more popular MovableType (which is what I use here).
That said, since instituting the changes I've made, I haven't had much problems with comment spam. What I have had, however, is ping floods where companies will hit my site six or seven pings per minute for 10 minutes at a time. Mostly they've been gambling sites with a sprinkling of the expected ED-type sites. None, of course, have been successful but it's interesting to look at the logs and see their attempts. What is not so good is the load these pings put on server but I don't know what else I can do other than get rid of trackback pings (which I'm not sure would make a difference).
In any case, the best defense I've seen seems to be to close comments and trackback pings after a length of time. I use seven days, which is why if you try to enter a comment after that time period your comment won't get through (hi Sjon! - ed.). But feel free to take a look at his work and decide for yourself what works best for you.
Aloha!
