I have many blogs, out of which Let's Sermo was getting 75 spam comments a day.
This was a headache.
So i decided to write a thread on how I reduced this and how you can protect your blog from spam.
1- You should have a plugin like Akismet, that catches the spam, at least most of it.
2-I noticed the spam comments. Most of them are coming from the same IP. So all you have to do is get a plugin like Wp-Ban and block that IP.
This will drastically reduce spam.
Hope this post helped you,