My b2evolution Version: Not Entered

I am getting a lot of spam being sent as comments to my blog. When I track back to the spamming sites, they too are blogs, just like mine. And they send inappropriate comments, about sucking eggs, or worse. How do I get this to go away? Is there a way to put one of those text verification things on it where the poster would have to type the text shown in a box?

Thank you.