One of my other blogs got targeted with spam comments, all aimed for one specific post. I did turn of comments for that post but that did not help, I have gotten 5,200 spams in a short period of time. Akismet properly catches the spam comments but I wanted to stop spam from getting into my blog at all. Today I found WP-SpamFree: A Powerful Anti-Spam Plugin for WordPress! which seems to solve my problem. After I installed WP-SpamFree there are no new spam comments.
For additional information, see Combating Comment Spam and Combating Comment Spam/Denying Access. Denying access is about blocking IP-numbers but in the spam comments I got they used a lot of different IP-numbers so it would not work.