Here are the steps:

1) Log into your blog account
2) From the Dashboard screen, click on the name of your blog
3) Click the link at the top of the screen titled Control Centre
4) On the Control Centre screen, click on the link titled AuthImage under the category "Anti spam management"
5) On the AuthImage Plugin screen, click the "enable" checkbox
6) click Update

This will require a user to enter a (given) code in order to post their comments. Spam generators can't see the code.