Hi,
You can prevent most of the comment spam by enabling the Anti Spam in the feature settings for the blog (click the gear icon next to the title of your blog). It will enable a captcha which makes it difficult for automated scripts to post spam and thats where most of it comes from. There are 3 different captchas, you can choose which one to use from Site Settings > Security tab > ANTI SPAM
You'll notice on my blog I'm using Recaptcha and I hardly ever get spam. Every now and then I get SEO spam which is where real people are posting comments just to get links to their site in hopes of improving their search engine rank. I see this occasionally in my blog and also in the forums. When I get the email notification about a comment in my blog it has the ip address of the comment poster, if I determine it is a spammer I ban the ip address in Admin Menu > Advanced Tools > Banned IP Addresses, and I delete the comment.
Hope it helps,
Joe