The blogs are very susceptible to automated blog spam. The only way to prevent it is to make everyone register to leave comments. You can sign up just to comment without having a blog. When they click to sign up it will let them choose to get a blog or just a username for commenting.
This option is configurable per blog account, so you can turn it off to let anyone comment on your blog without registering. If you start to get spam, you can turn it back on. Go to your blog admincp and click the Options tab. It's the first page there for General Options. Just below where you can set your blog title, you'll see a check box next to Membership. It's checked on by default. If you uncheck it then people can comment on your blog without registering.
There are more options for commenting that you can explore. Still under Options, choose the Discussion tab and check them out
I hope that helps