If you want to protect your blog with spam comments than you should try Akismet which can help you a lot to prevent spam comments on your blog.Here is a simple step to get a free Akismet API key for your blog.
1.Go to Akismet and register your new account.
2.Now choose and click the “Personal Plan” than see “WHAT IS AKISMET WORTH TO YOU?” and drag the button to zero (0).
3.After singing up you will get Akismet API key in your Email Address.
4.Finally you are done.