These steps will help you prevent users and robots from posting Spam on your WordPress blog.

1. Inside the WordPress Dashboard, select Settings, then Discussion in the navigation bar on the left.
   
2. From the Discussion page, scroll down.
3. Check the box next to Users must be registered and logged in to comment.
   
4. Check the box next to Comment must be manually approved.
   
5. You can enter words, phrases, or content into a box that will hold comments for you to moderate them.
   
6. You can enter words, phrases, or content into a box that will delete comments containing these things.
   
7. Scroll down to the bottom and click Save Changes.
   

Your WordPress blog has now been approved to fight against Spam.